README: move "custom branch" part to "Basic use"
We should distinguish better between documentation for the use of the
git repositories and for the development of them, so that new users
won't get confused by the detailed description of the special
proceedings.
So move the description of how to create and use a custom branch out of
the "Advanced use" into the "Basic use" chapter and rename these two
chapters to "For developers" resp. "For users".
Also adjust the five moved lines of the description a bit and add some
description to the "For developers" chapter.