Add "test" script + instructions

[ljharb]

I figured out that mocha will run the tests, but 5 of them fail with "Error: Forbidden". Do I have to set up environment variables for the tests to pass?

It looks like GITHUB_OAUTH_TOKEN, GITHUB_USERNAME, and GITHUB_PASSWORD are what the tests expect, but that should probably be documented (and there should probably be explicit tests that fail when these env vars are not defined.

[ljharb]

Also, my github account uses 2FA - does that mean it can't work with this lib?

[JuanCaicedo]

@ljharb for what it's worth, I think if you have a github oauth token you don't need to provide them with 2FA. I'm working with this problem now as well 🙂