Add configuration options to README

In order to make it clearer to users that `scss-lint` is customizable,
include the configuration options in the README.

Change-Id: Id48ea992ed2a7e04772239c9b7eb30b1b16b979e
Reviewed-on: https://gerrit.causes.com/33673
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>