Improve the rebase docs in contributing guidelines
Improve the rebase docs in contributing guidelines

These docs do not tell people how to deal with branch diversions which
happen as a result of a rebase, and could lead (let's be real, probably
_has led_) to users pulling/merging, which will add a buttload of
unrelated commits to their pull request.

This update shows how to force-push when necessary.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt