Mention best practice of using multiple `--join` arguments in a cluster

It is usually advisable to give rethinkdb multiple --join arguments, one for each other server in the cluster.
That way, if one other server is down when restarting a RethinkDB node, it will still be able to connect to the cluster rather than waiting (possibly forever) for that one server to come up.

A RethinkDB server started with multiple --join options will become available once it's able to connect to at least one of them. If it can't connect to any of the listed servers, it will stay in a partially available state and keep trying to connect. In this state it can respond to incoming cluster connections from other nodes, but will not accept any client connections nor process queries.

http://rethinkdb.com/docs/start-a-server/ might be the best way to put this (under "A RethinkDB cluster using multiple machines").

This came up on our community Slack channel.