Update README.markdown

As a follow-up to our email conversation, these revisions change the README file into a Markdown file, so that it will have clickable links.

I've added a note near the top about the addition of the domain separator bits in the draft FIPS-202 standard. Though I think it was always clear that domain separator bits would be added, people still seem somewhat confused about this issue -- see, e.g., https://github.com/CakeML/mitb/issues/1. I've also added a link to the very helpful macros in KeccakHash.h which I somehow missed!