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>

COLLABORATOR_GUIDE.md

@@ -212,3 +212,14 @@ Time to push it:
 ```text
 $ git push origin v1.x
 ```
+
+### I just made a mistake
+
+With git, there's a way to override remote trees by force pushing
+(`git push -f`). This should generally be seen as forbidden (since
+you're rewriting history on a repository other people are working
+against) but is allowed for simpler slip-ups such as typos in commit
+messages. However, you are only allowed to force push to any io.js
+branch within 10 minutes from your original push. If someone else
+pushes to the branch your commit lives in or the 10 minute period
+passes, consider the commit final.