Add link to `overcommit` gem

An extensible Git hook manager, with built-in support for JSHint. Plus a
lot more:

  http://causes.github.io/blog/2013/05/30/overcommit-the-opinionated-git-hook-manager/
Merge pull request #96 from sectioneight/add-link-to-overcommit

Add link to `overcommit` gem