`README.txt`→`README.md` (use markdown for README)

Markdown is designed to be the best of both worlds—perfectly readable as plain text, but also a 1:1 mapping to HTML.  Beats the hell out of `*.txt` any day.  ☺
Minor edits to `README.md`

Tweaked for style, clarity, formatting, and readability
Added `README.txt` symlink for non-markdown luddites…just in case ☺