Add docs badge to README

Add a docs badge to the README to show off inline-documentation to
potential contributors.

The badge links to Inch CI, a project that tries to raise the visibility
of inline-docs to encourage aspiring Rubyists to document their code.

Status page is available at http://inch-ci.org/github/causes/scss-lint/

Change-Id: I03485dc4ef8363f0d71fe55f930a0df631fed040
Reviewed-on: http://gerrit.causes.com/40374
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>