Add link to blog post about custom hooks

This will provide more examples on how to write custom hooks for
Overcommit.

Change-Id: Iab573d5b64e67307e6cc5694dc75ac49810da79d
Reviewed-on: http://gerrit.causes.com/41667
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Tested-by: Shane da Silva <shane.dasilva@brigade.com>