[polari] docs: Add README section for default branch

[Florian Müllner <fmuellner src gnome org>]

commit 2faf679176a1e8533c876c48e73c416df7ae39ee
Author: Florian Müllner <fmuellner gnome org>
Date:   Tue Jul 13 21:49:54 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)
    
    https://gitlab.gnome.org/GNOME/polari/-/merge_requests/202

 README.md | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
---
diff --git a/README.md b/README.md
index 34619a2e..d3ad686e 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,20 @@ with nice folks, please join us in [#polari on irc.gnome.org][irc-channel].
 If you found a problem or have a feature suggestion, please report the
 issue to the GNOME [bug tracking system][bug-tracker].
 
+
+## Default branch
+
+The default development branch is `main`. If you still have a local
+checkout with 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
+```
+
 [project-page]: https://wiki.gnome.org/Apps/Polari
 [build-instructions]: https://wiki.gnome.org/Newcomers/BuildProject
 [irc-channel]: irc://irc.gnome.org/%23polari