Document how to use JWT instead of OAuth

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](http://2ality.com/2015/10/google-analytics-api.html) and after that provide it to the `GuessPlugin` [configuration object](https://github.com/guess-js/guess/blob/master/packages/guess-webpack/src/guess-webpack.ts#L17).

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