Update CHANGELOG

Add documentation of the new features added to `scss-lint`.

Change-Id: Ic5189c724a2267ef68cc5ce9fa1d34c83708a13f
Reviewed-on: https://gerrit.causes.com/30931
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>