Include creation of branch and provide name in docs

Before submitting changes for an import branch in the target gerrit,
need to ensure there is a branch in the remote created from the correct
base point. Additionally need to supply a branch name when uploading for
review against a non-default branch.

'git-upstream' will create a 'base' branch, specifically to provide a
reference to the starting point of the new import. This can be used to
push up the correct starting point of the new import, before uploading
changes for review.

Change-Id: If80057ad32838380d521d30609ead455d6905d2a