Credit the original package, add usage etc.

[dandv]

I'm happy to see someone took initiative to just get the TypeScript rewrite done when it was "out of the question" for the original project.

But seeing that this rewrite has had no traction so far, is not encouraging.

Some ideas:

1. Credit the original package in the README.
2. Add some documentation.

[webbrain-one]

I opened #27, which adds credit to the original validator.js package and a short usage section to the README. Thanks!