update rit github update repo README file

Signed-off-by: GuillaumeFalourd <[email protected]>

Author: GuillaumeFalourd

github/update/repo/README.md

@@ -15,6 +15,9 @@ rit github update repo
 
 This formula allows the user to update the default branch on all his repositories.
 
+- if the new default branch already exists on the repository, it is set as default directly.
+- if the new default branch doesn't exists on the respository yet, it is created based on another branch (reference), before being set as default.
+
 ## Demo
 
 ### Sample 1