add doc for maintenance branches

I am getting:
[master]schubi@g155:autoinstallation (1) $ git checkout Code-11-SP3
error: pathspec 'Code-11-SP3' did not match any file(s) known to git.

Should it be :

 git checkout -t -b Code-11-SP3-L3-bug-898525 remotes/origin/Code-11-SP3