Add Markdownlint to CI

[dylanhitt]

What are the feelings about implementing markdownlint. This could be pretty useful our documentation grows?

Same pattern as other things probably will put a check in the CI to ensure nothing slips through. I'll probably add a target to the Makefile called lint-markdown then add a catch all lint target that is lint-all: lint-markdown lint for local checking.

[EwenQuim]

Good idea!

OK for the Makefile target, I think that's a good idea to keep linters and all that way.

[dylanhitt]

Completed by #138