Update README with linter documentation

Add some documentation describing the checks that `haml-lint` performs,
along with their justification.

Also sneaking in here are some additional README updates for the
command-line flags and linking to `overcommit`.

Change-Id: I0990791be557665626a6f467e0cdc7df1c666272
Reviewed-on: https://gerrit.causes.com/31047
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>