Formatted readme as markdown

Hi and thanks for maintaining a great tool!

I've found the plaintext readme is hard to read, so I reformatted it in Markdown. The diff is gigantic but I haven't changed any of the content whatsoever. I've also refrained from using the fancier features of Markdown to keep it readable for those who do not view it on Github or through a parser.

I think that this makes it a lot more pleasant to read on Github without making it less pleasant for those who don't.

Cheers!