Add Requirements section to README

Since scss-lint has a hard dependency on using Ruby 1.9, we should make
it clear that this is a requirement. Additionally, state that only Sass
3 syntax is supported.

This addresses the concerns raised in #16.

Change-Id: I4e00ab287077ac0d5be33c81e4e2a21bfdcdc58f
Reviewed-on: https://gerrit.causes.com/23057
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Kim Douglas Clarence Mason <kim.mason@causes.com>