Add YARD files to .gitignore

The doc directory contains the compiled YARD documentation, which can
exist after running:

  yard doc

The .yardoc directory is used by the YARD server, which can exist after
running:

  yard server

or:

  yard server --reload

In either case, we don't want these in our repo. Our YARD documentation
is being hosted by rdoc.info:

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

Change-Id: Ia873b7c114eb699b29baace2f68a11e00d8568dd
Reviewed-on: http://gerrit.causes.com/36076
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>