[gtk] Update CONTRIBUTING.md

[Emmanuele Bassi <ebassi src gnome org>]

commit 27abc9fefece5ab08decefeb1949c62837a02b11
Author: Emmanuele Bassi <ebassi gmail com>
Date:   Tue Mar 13 18:13:46 2018 +0000

    Update CONTRIBUTING.md
    
    Update the instructions to match the GitLab workflow.

 CONTRIBUTING.md | 45 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 34 insertions(+), 11 deletions(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d9cb111810..8438372a7e 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -1,7 +1,7 @@
 If you want to hack on the GTK+ project, you'll need to have the development
 tools appropriate for your operating system, including:
 
-       - Python (2.7 or 3.x)
+       - Python 3.x
        - Meson
        - Ninja
        - Gettext (19.7 or newer)
@@ -12,19 +12,30 @@ can be found here:
 
         https://developer.gnome.org
 
-Information about using git with GNOME can be found here:
+Information about using GitLab with GNOME can be found here:
 
-        https://wiki.gnome.org/Git
+        https://wiki.gnome.org/GitLab
 
 In order to get Git GTK+ installed on your system, you need to have the
 required versions of all the GTK+ dependencies; typically, this means a
-recent version of GLib, Cairo, Pango, and ATK.
+recent version of GLib, Cairo, Pango, and ATK, as well as the platform
+specific dependencies for the windowing system you are using (Wayland, X11,
+Windows, or macOS).
 
-**Note**: if you plan to push changes to back to the master repository and
-have a GNOME account, you want to use the following instead:
+You should start by forking the GTK repository from the GitLab web UI, and
+cloning from your fork:
+
+```ssh
+$ git clone https://gitlab.gnome.org/yourusername/gtk.git
+$ cd gtk
+```
+
+**Note**: if you plan to push changes to back to the main repository and
+have a GNOME account, you can skip the fork, and use the following instead:
 
 ```sh
-$ git clone ssh://<username>@git.gnome.org/git/gtk+
+$ git clone git gitlab gnome org:GNOME/gtk.git
+$ cd gtk
 ```
 
 To compile the Git version of GTK+ on your system, you will need to
@@ -32,9 +43,21 @@ configure your build using Meson:
 
 ```sh
 $ meson _builddir .
+$ cd _builddir
+$ ninja
+```
+
+**Note**: For information about submitting patches and pushing changes
+to Git, see the `README.md` and `README.commits` files. In particular,
+don't, under any circumstances, push anything to Git before reading and
+understanding `README.commmits`.
+
+Typically, you should work on your own branch:
+
+```sh
+$ git checkout -b your-branch
 ```
 
-For information about submitting patches and pushing changes to Git, see the
-`README.md` and `README.commits` files. In particular, don't, under any
-circumstances, push anything to Git before reading and understanding
-`README.commmits`.
+Once you've finished working on the bug fix or feature, push the branch
+to the Git repository and open a new merge request, to let the GTK
+maintainers review your contribution.
\ No newline at end of file