Fix README highlighting and structure, and use `.md`

Hi again, Tom. :smile:

I initially just wanted to fix the Meteorite link, but got a bit carried away.

I guess the API docs could still use some help, but at least this gets things started.

Cheers!

Oh: I intentionally put the `.markdown` to `.md` renaming in a separate commit so you can drop that if `.markdown` is your preference. It's just me being pedantic :stuck_out_tongue: