[mutter] README: Add link to wiki

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

commit 5ab187da701f2a8817fe47700c86bbae08daf4e5
Author: Jonas Ådahl <jadahl gmail com>
Date:   Fri Oct 23 20:43:40 2020 +0200

    README: Add link to wiki
    
    The wiki has gotten a few pages recently, lets link to it.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1521

 README.md | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)
---
diff --git a/README.md b/README.md
index 964b04d467..298fb87a66 100644
--- a/README.md
+++ b/README.md
@@ -26,11 +26,16 @@ debugging purposes.
 
 To contribute, open merge requests at https://gitlab.gnome.org/GNOME/mutter.
 
+It can be useful to look at the documentation available at the
+[Wiki](https://gitlab.gnome.org/GNOME/mutter/-/wikis/home).
+
 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.
+with some minor additions such as preferring regular C types and
+`stdint.h` types over GLib fundamental types, except for gboolean, and
+guint/gulong for GSource ids and signal handler ids. There is also 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