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>

1 files changed, 13 insertions, 0 deletions

diff --git a/README.md b/README.md
index b44dc057e..62c894407 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,