Note extra functionality in readme

In addition to commit-msg and pre-commit (the two that are currently
Ruby-based and extensible), `overcommit` also ships with additional
hooks. This mentions them and briefly explains the purpose in the
readme.

Change-Id: I5cd1538684bd4235763f9bb61cc1f1fc90a1df01
Reviewed-on: https://gerrit.causes.com/23106
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Aiden Scandella <aiden@causes.com>