Add docs in README.md

[ty-everett]

This pull request completely trashes the README.md file for this repository.

But only because, for better or worse, the API documentation for the current version of this library is—quite frankly—trash.

However, I think we can and should definitely improve it. Perhaps, if people are forced to look at it, they will be more motivated to update and improve it.

I don't know if we should merge this into master now, or keep it in its own branch until after the documentation can be improved. It will be a massive effort, but if we want people to upgrade from bsv@^1.5.5 it will need to be done.

While I am happy to help with the documentation effort itself as I have the time, I lack intimate familiarity with this library, and have yet to use it in my applications (due to lack of quality documentation). While the code for bsv@2 might be 1,000 times better than bsv@1, my view is that this does not matter until people can reference and use it. One of the reasons I use bsv@1 is because I can simply visit docs.moneybutton.com and see all the great examples.

Thoughts and improvements welcome. We should discuss whether it makes sense to merge this now, or keep it on its own branch until the documentation effort moves forward.