Document Firehose protocol in PROTOCOL.md

[bradgessler]

Document Firehose protocol for publishers and subscribers over HTTP Long Polling and WebSockets in PROTOCOL.md. Ideally the message formats should:

• Be kept as consistent as possible between publishing and consuming
• Remain as consistent as possible with its representation on the server https://github.com/firehoseio/firehose/blob/master/lib/firehose/server/message_buffer.rb#L10 and the wire format
• Remain consistent over its transport (e.g. web sockets is a message per frame, long polling is 1 message payload per HTTP request, batch subscriptions over HTTP long polling is a JSON array of messages.

I propose the following format for publishing:

// Message from service published to Firehose
{
  "message": "Bee boop",
  "channel": "/greetings/from/mars",
  "ttl": 60
}

And the following for a client consuming a sequence of messages:

// Message from Firehose to consuming client
{
  "message": "Bee boop",
  "channel": "/greetings/from/mars",
  "sequence": 101
}