[mutter] README: Add contribution section

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit f3dd97e67b778e0a510b989a820bd3e0820449e6
Author: Jonas Ådahl <jadahl gmail com>
Date:   Wed Feb 6 11:41:43 2019 +0100

    README: Add contribution section
    
    Short and incomplete blurb about coding style and commit message
    guidelines.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/440

 README.md | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
---
diff --git a/README.md b/README.md
index 56141d1a2..964b04d46 100644
--- a/README.md
+++ b/README.md
@@ -22,6 +22,20 @@ by  Gala, elementary OS's window manager. It can also be run standalone, using
 the  command "mutter", but just running plain mutter is only intended for
 debugging purposes.
 
+## Contributing
+
+To contribute, open merge requests at https://gitlab.gnome.org/GNOME/mutter.
+
+The coding style used is primarily the GNU flavor of the [GNOME coding
+style](https://developer.gnome.org/programming-guidelines/stable/c-coding-style.html.en)
+with some minor additions such as preferring `stdint.h` types over GLib
+fundamental types, and a soft 80 character line limit. However, in general,
+look at the file you're editing for inspiration.
+
+Commit messages should follow the [GNOME commit message
+guidelines](https://wiki.gnome.org/Git/CommitMessages). We require an URL
+to either an issue or a merge request in each commit.
+
 ## License
 
 Mutter is distributed under the terms of the GNU General Public License,