Document basic backoff protocol.

Here is a super-basic backoff protocol to go along with the 429/503 error responses.  It only does retry-after, no proof-of-work shortcuts allowed.  We can expand with a PoW thing later, but let's get the basics into the protocol so clients can bake in support as early as possible.

It's not clear how much value we'll get out of the `Backoff` header for this service, since FxA clients wont tend to be doing lots of background requests to the server.  But I think it's better to have a consistent mechanism across all services, so we might as well start by including it in here.

@ckarlof r?