Include a git-push example for creating a remote branch

Include a git-push example for creating a remote branch

Many users get confused when `git push origin master:foo` works
when foo already exists on the remote repository but are confused
when foo doesn't exist as a branch and this form does not create
the branch foo.

This new example highlights the trick of including refs/heads/
in front of the desired branch name to create a branch.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>