[gtk/commit-docs] docs: Move commit style docs to the contribution guide

[Emmanuele Bassi <ebassi src gnome org>]

commit 1c5c1ad2fd4625c8287e752e3f376fc78d1b2b72
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Wed Sep 26 14:53:32 2018 +0100

    docs: Move commit style docs to the contribution guide
    
    There's no point in having a separate file detailing how commits ought
    to work, considering we already have a contribution guide.

 CONTRIBUTING.md   | 85 +++++++++++++++++++++++++++++++++++++++++++++++++++----
 README.commits.md | 71 ----------------------------------------------
 2 files changed, 79 insertions(+), 77 deletions(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 3326f99043..733b2bece1 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -25,7 +25,7 @@ Windows, or macOS).
 You should start by forking the GTK repository from the GitLab web UI, and
 cloning from your fork:
 
-```ssh
+```sh
 $ git clone https://gitlab.gnome.org/yourusername/gtk.git
 $ cd gtk
 ```
@@ -47,11 +47,6 @@ $ cd _builddir
 $ ninja
 ```
 
-**Note**: For information about submitting patches and pushing changes
-to Git, see the [README.md](./README.md) and [README.commits.md](./README.commits.md) files. In particular,
-don't, under any circumstances, push anything to Git before reading and
-understanding [README.commits.md](./README.commits.md).
-
 Typically, you should work on your own branch:
 
 ```sh
@@ -63,3 +58,81 @@ to the Git repository and open a new merge request, to let the GTK
 maintainers review your contribution. The [CODE-OWNERS](./docs/CODE-OWNERS)
 document contains the list of core contributors to GTK and the areas for
 which they are responsible.
+
+### 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.
+```
+
+ - 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. Conside the commit
+ message as an email sent to the developers (or yourself, six months
+ down the line) detailin **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 to
+ automatically close the issue on push:
+
+```plain
+Closes #1234
+```
+
+  or:
+
+```plain
+Fixes #1234
+```
+
+  or:
+
+```plain
+Closes https://gitlab.gnome.org/GNOME/gtk/issues/1234
+```
+
+### Access to the GTK repository
+
+GTK+ is part of the GNOME infrastructure. At the current time, any
+person with write access to the GNOME repository, can make changes to
+GTK+. This is a good thing, in that it encourages many people to work
+on GTK+, and progress can be made quickly. However, GTK+ is a fairly
+large and complicated package that many other things depend on, so to
+avoid unnecessary breakage, and to take advantage of the knowledge
+about GTK+ that has been built up over the years, we'd like to ask
+people committing to GTK+ to follow a few rules:
+
+0. Ask first. If your changes are major, or could possibly break existing
+ code, you should always ask. If your change is minor and you've
+ been working on GTK+ for a while it probably isn't necessary
+ to ask. But when in doubt, ask. Even if your change is correct,
+ somebody may know a better way to do things.
+ If you are making changes to GTK+, you should be subscribed
+ to [gtk-devel-list](https://mail.gnome.org/mailman/listinfo/gtk-devel-list).
+ This is a good place to ask about intended changes.
+ `#gtk+` on GIMPNet (irc.gnome.org) is also a good place to find GTK+
+ developers to discuss changes, but if you live outside of the EU/US time
+ zones, email to gtk-devel-list is the most certain and preferred method.
+
+0. Ask _first_.
+
+0. Always write a meaningful commit message. Changes without a sufficient
+ commit message will be reverted.