Markdown documentation

I took a page out of [Atom's](https://github.com/atom/atom/tree/master/docs) book by converting the docs into Markdown.

This means that they're much easier to maintain and you can also edit and read them right on GitHub ([example](https://github.com/nixxquality/ffms2/blob/master/doc/ffms2-api.md)).

I understand that you probably want to ship .html files in the release .zips, but I'm pretty sure it should be easy to add a Markdown renderer to the build process. If the makefiles for the build .zips are in the repo, I can add it myself.