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

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

commit b58f057713b15c62ab16e628918f90dfcfb38373
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed Jul 14 21:10:23 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/-/merge_requests/1914>

 README.md | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/README.md b/README.md
index 7420bc3ac8..523ff3ad7c 100644
--- a/README.md
+++ b/README.md
@@ -20,6 +20,19 @@ 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.
 
+## 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
 GNOME Shell is distributed under the terms of the GNU General Public License,
 version 2 or later. See the [COPYING][license] file for details.