Generate a starter configuration file on install

In order to ease new users into using Overcommit, generate a starter
configuration file with some useful instructions and links if an
`.overcommit.yml` file does not yet exist in the repository.

Change-Id: I20ed60440bdc4dcb8f2b80223bc81aa23f1243c9
Reviewed-on: http://gerrit.causes.com/40980
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>