Mention main in NEWS and README.md

README.md

@@ -116,6 +116,20 @@ docs/reference/gtk/html/gtk-building.html
 
 Or [online](https://docs.gtk.org/gtk4/building.html)
 
+Default branch renamed to `main`
+--------------------------------
+
+The default development branch of GTK has been renamed to `main`.
+To update your local checkout, 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
+```
+
 How to report bugs
 ------------------