[mutter] docs: Add README section for default branch

[Marge Bot <marge-bot src gnome org>]

commit 7862f143937e43dca0513af3a24dabfb4d0db4fc
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed Jul 14 21:14:32 2021 +0200

    docs: Add README section for default branch
    
    We are about to change it, so briefly outline how to update local
    checkouts.
    
    (Copied from glib)
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1930>

 README.md | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/README.md b/README.md
index b44dc057e4..62c8944072 100644
--- a/README.md
+++ b/README.md
@@ -42,6 +42,19 @@ commit subjects with a relevant topic, such as `compositor:` or
 `clutter/actor:`, and it's always better to write too much in the commit
 message body than too little.
 
+## Default branch
+
+The default development branch is `main`. If you still have a local
+checkout under the old name, use:
+```sh
+git checkout master
+git branch -m master main
+git fetch
+git branch --unset-upstream
+git branch -u origin/main
+git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main
+```
+
 ## License
 
 Mutter is distributed under the terms of the GNU General Public License,