Update README describing how configure SCSS Lint
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 <d95b56ce41a2e1ac4cecdd398defd7414407cc08@causes.com>
Reviewed-by: Joe Lencioni <1dd72cf724deaab5db236f616560fc7067b734e1@causes.com>

baileyparker/scss-lint,ivantsepp/scss-lint,dwayhs/scss-lint,chatzipan/scss-lint,golmansax/scss-lint,bkeepers/scss-lint,gajus/scss-lint,ingdir/scss-lint,davecarter/scss-lint,gatero/scss-lint,cih/scss-lint,philipgiuliani/scss-lint,leseulsteve/scss-lint,teoljungberg/scss-lint