[gnome-shell-extensions] docs: Add README section for default branch

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

commit 7ba9b87064a37761ed819a33147aae1e5ab8f300
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed Jul 14 21:11:59 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/gnome-shell-extensions/-/merge_requests/177>

 README.md | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/README.md b/README.md
index 3b84cb1..fb8be6b 100644
--- a/README.md
+++ b/README.md
@@ -69,6 +69,19 @@ GSettings key.
 
      Adds a simple workspace switcher to the top bar.
 
+## Default branch
+
+The default development branch is `main`. If you still have a local
+checkout under the old name, use:
+```sh
+git checkout matser
+git branch -m master main
+git fetch
+git branch --unset-upstream
+git branch -u origin/master
+git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main
+```
+
 ## License
 
 GNOME Shell Extensions are distributed under the terms of the GNU General