Add link to library this is based on to docs

[mikemaccana]

From README.md:

BigNum in pure javascript

This seems to indicate this is a port of an existing library (hence no docs for new BigNum() etc - as we can read the docs on the original) - but which BigNum on the internet is the original one? There's too many existing libraries that come up in a search.

Basically: can we just make it:

BigNum in pure JavaScript

?