Update README describing how configure SCSS Lint

Add some documentation explaining the configuration system as well as
a list of supported command line flags.

Also sneaking in are an additional link to `overcommit` at the top of
the README (when referring to SCM hooks) and capitalization of a
heading.

Change-Id: I5178565a010e55a8a3ab373a5198f2fea25284b3
Reviewed-on: https://gerrit.causes.com/30930
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>