[gdk-pixbuf] docs: Update the contribution guide

[Emmanuele Bassi <ebassi src gnome org>]

commit 5a670c403ea477405b4a3889169dfa4a79b2d52f
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Thu Feb 28 16:09:12 2019 +0000

    docs: Update the contribution guide
    
    Bring it in line with GTK's and GLib's contribution guides, and make it
    a bit more friendly and accessible for newcomers.

 CONTRIBUTING.md | 144 +++++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 131 insertions(+), 13 deletions(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index feb7622a2..d3fd1f006 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -1,6 +1,86 @@
 Contributing to GdkPixbuf
 =========================
 
+Thank you for considering contributing to GdkPixbuf!
+
+These guidelines are meant for new contributors, regardless of their level
+of proficiency; following them allows the core developers of GdkPixbuf to
+more effectively evaluate your contribution, and provide prompt feedback to
+you. Additionally, by following these guidelines you clearly communicate
+that you respect the time and effort that the people developing GdkPixbuf
+put into managing the project.
+
+GdkPixbuf is a free software utility library, and it would not exist without
+contributions from the free and open source software community. There are
+many things that we value:
+
+ - bug reporting and fixing
+ - documentation and examples
+ - tests
+ - testing and support for other platforms
+ - new features
+
+Please, do not use the issue tracker for support questions. If you have
+questions on how to use GdkPixbuf effectively, you can use:
+
+ - the `#gtk+` IRC channel on irc.gnome.org
+ - the [gtk](https://mail.gnome.org/mailman/listinfo/gtk-list) mailing list,
+   for general questions on GdkPixbuf
+ - the [gtk-devel](https://mail.gnome.org/mailman/listinfo/gtk-devel-list)
+   mailing list, for questions on developing GdkPixbuf itself
+
+The issue tracker is meant to be used for actionable issues only.
+
+GdkPixbuf is a library with a long history, and it has been incrementally
+modified over years, so it may retain some older coding practices alongside
+newer ones.
+
+As it deals with loading image data into user processes, it's also important
+to note that GdkPixbuf must always deal with potential security issues.
+
+## How to report bugs
+
+### Security issues
+
+You should not open a new issue for security related questions.
+
+When in doubt, send an email to the [security](mailto:security gnome org)
+mailing list.
+
+### Bug reports
+
+If you’re reporting a bug make sure to list:
+
+ 1. which version of GdkPixbuf (and its dependencies) are you using?
+ 2. which operating system are you using?
+ 3. the necessary steps to reproduce the issue
+ 4. the expected outcome
+ 5. a description of the behavior
+ 6. a small, self-contained example exhibiting the behavior
+
+If the issue includes a crash, you should also include:
+
+ 1. the eventual warnings printed on the terminal
+ 2. a backtrace, obtained with tools such as GDB or LLDB
+
+If the issue includes a memory leak, you should also include:
+
+ - a log of definite leaks from a tool such as [valgrind’s 
memcheck](http://valgrind.org/docs/manual/mc-manual.html)
+
+For small issues, such as:
+
+ - spelling/grammar fixes in the documentation,
+ - typo correction,
+ - comment clean ups,
+ - changes to metadata files (CI, `.gitignore`),
+ - build system changes, or
+ - source tree clean ups and reorganizations;
+
+or for self-contained bug fixes where you have implemented and tested a solution
+already, you should directly open a merge request instead of filing a new issue.
+
+## Your first contribution
+
 ### Requirements
 
 If you wish to contribute to GdkPixbuf you will need to install the
@@ -12,7 +92,7 @@ appropriate development tools for your operating system, including:
  - Gettext
  - a C99 compatible compiler
 
-### Cloning and building
+### Getting started
 
 You can start by cloning the Git repository:
 
@@ -29,8 +109,6 @@ $ cd _build
 $ ninja
 ```
 
-### Development
-
 Once you built GdkPixbuf, you should create a new branch in order
 to work on your bug fix, or your feature, undisturbed:
 
@@ -61,7 +139,56 @@ $ ninja
 $ ninja gdk-pixbuf-doc
 ```
 
-### Contributing
+### Commit messages
+
+The expected format for git commit messages is as follows:
+
+```plain
+Short explanation of the commit
+
+Longer explanation explaining exactly what’s changed, whether any
+external or private interfaces changed, what bugs were fixed (with bug
+tracker reference if applicable) and so forth. Be concise but not too
+brief.
+
+Closes #1234
+```
+
+ - Always add a brief description of the commit to the _first_ line of
+ the commit and terminate by two newlines (it will work without the
+ second newline, but that is not nice for the interfaces).
+
+ - First line (the brief description) must only be one sentence and
+ should start with a capital letter unless it starts with a lowercase
+ symbol or identifier. Don’t use a trailing period either. Don’t exceed
+ 72 characters.
+
+ - The main description (the body) is normal prose and should use normal
+ punctuation and capital letters where appropriate. Consider the commit
+ message as an email sent to the developers (or yourself, six months
+ down the line) detailing **why** you changed something. There’s no need
+ to specify the **how**: the changes can be inlined.
+
+ - When committing code on behalf of others use the `--author` option, e.g.
+ `git commit -a --author "Joe Coder <joe coder org>"` and `--signoff`.
+
+ - If your commit is addressing an issue, use the
+ [GitLab syntax](https://docs.gitlab.com/ce/user/project/issues/automatic_issue_closing.html)
+ to automatically close the issue when merging the commit with the upstream
+ repository:
+
+```plain
+Closes #1234
+Fixes #1234
+Closes: https://gitlab.gnome.org/GNOME/glib/issues/1234
+```
+
+ - If you have a merge request with multiple commits and none of them
+ completely fixes an issue, you should add a reference to the issue in
+ the commit message, e.g. `Bug: #1234`, and use the automatic issue
+ closing syntax in the description of the merge request.
+
+### Submitting your contribution for review
 
 Once you're done with your work, you should commit it, push it to a remote
 repository, and open a Merge Request against the GdkPixbuf upstream
@@ -120,15 +247,6 @@ GdkPixbuf is divided into logical sections:
  - The [animated image][gdkpixbuf-api-animation] API, for image formats
    that support animations
 
-### Notes
-
-GdkPixbuf is a library with a long history, and it has been incrementally
-modified over years, so it may retain some older coding practices alongside
-newer ones.
-
-As it deals with loading image data into user processes, it's also important
-to note that GdkPixbuf must always deal with potential security issues.
-
 [gdkpixbuf-api-core]: https://developer.gnome.org/gdk-pixbuf/stable/gdk-pixbuf-The-GdkPixbuf-Structure.html
 [gdkpixbuf-api-ctor]: https://developer.gnome.org/gdk-pixbuf/stable/gdk-pixbuf-Image-Data-in-Memory.html
 [gdkpixbuf-api-load]: https://developer.gnome.org/gdk-pixbuf/stable/gdk-pixbuf-File-Loading.html