Generate plaintext README

[israel-lugo]

README.md looks nice in Github, but it's not practical for users who download our code, or install the package.

A README.txt can be generated using pandoc:
pandoc -t plain -f markdown README.md > README.txt

But this doesn't support all formatting. Namely, it doesn't support HTML <sup> tags, which we use to represent 2⁶⁴, leaving us with "1 in 264" instead of "1 in 2⁶⁴". And that's just not the same thing.

We could change to AsciiDoc. That supports superscripting natively.