Document how to use JWT instead of OAuth

[mgechev]

Often folks want to run Guess.js inside CI where the only way to authenticate is using an authentication token. Although this behavior is supported, it's not well documented.

To authenticate with a JWT, first one should generate a token and after that provide it to the GuessPlugin configuration object.

Consider writing a guide on building app with the GuessPlugin in CI or simply, a guide on how to authenticate with a token.