Add link to rdoc.info documentation

This website hosts compiled YARD documentation. Although there isn't
much YARD in this project yet, this documentation serves as a good
starting point to build off of.

At the same time, I set up a Github service hook to automatically
regenerate the documentation at rdoc.info every time a new commit is
pushed to master.

The documentation can be found at:

  http://rdoc.info/github/causes/scss-lint/master/frames

Change-Id: I371ac87488dc14321c86c9cf680b28dfa5c5c9de
Reviewed-on: http://gerrit.causes.com/36075
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>