doc: Document forced pushing with git

Mention that we generally disallow forced pushes but allow it
in trivial cases within 10 minutes of the original push unless
the branch pushed to already has new commits.

PR-URL: https://github.com/iojs/io.js/pull/1420


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>