document the firebase API

[seiyria]

After #20 is done, this should be done as well, so people understand the API and how it's structured.

[seiyria]

Also, to allow people to do it, maybe issue a private key (regeneratable) based on this.

[seiyria]

From one of the ops in #firebase:

[18:22:03] <mikelehen> seiyria: Not sure if this works for you, but after authenticating to firebase, the authData has a token you can get (e.g authData.token from JS) and you could send that to the REST API with ?auth=<token>
[18:23:01] <mikelehen> That token is subject to the "Session Length" you have configured in your Firebase dashboard though.
[18:23:16] <mikelehen> So it'll expire at some point and they'd need to get a new one.

more info here