README: small updates. (#166)
README: small updates. (#166)

* Point to the draft repo instead of a specific version (this is largely
to accommodate POST-as-GET and saves us having to keep bumping the draft
number)
* Fix a reference to RabbitMQ/Boulder that is no longer true.
letsencrypt/pebble,shred/pebble,letsencrypt/pebble,letsencrypt/pebble,shred/pebble