Also added README.md and CONTRIBUTING.md to the linter.
Note, that I am still getting one failer (in two locations). However
I consider that failer a bug in the linter and have reported it
upstream. We could disable that Rule (MD031), but as we are not
requiring the lint rules to pass presently, I just left it alone.
Also, while the code linter is set to allow lines 119 chars long,
I am using the Markdown linter's default of 80. Prose is easier to
read with shorter line lenghts, so I think it makes more sense to
use the default. Also, changing the default would have required
adding a config file. Adding a Ruby file for only one minor setting
seems silly, so I left it alone.
