[gnome-shell] README.md: Add contribution section

[Florian Müllner <fmuellner src gnome org>]

commit 382282b931bf599b8e607cdf912dc952f7816986
Author: Jonas Ådahl <jadahl gmail com>
Date:   Wed Feb 13 20:30:21 2019 +0100

    README.md: Add contribution section
    
    Include a link to where to create merge requests, and describe the
    requirements regarding commit messages which we are about to enforce.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/410

 README.md | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/README.md b/README.md
index 137faa602..a6bbb0a61 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,14 @@ see the [project wiki][project-wiki].
 
 Bugs should be reported to the GNOME [bug tracking system][bug-tracker].
 
+## Contributing
+
+To contribute, open merge requests at https://gitlab.gnome.org/GNOME/gnome-shell.
+
+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
 GNOME Shell is distributed under the terms of the GNU General Public License,
 version 2 or later. See the [COPYING][license] file for details.