Request: Read Replica documentation

There is no current documentation showing how to set up read replicas in Citus Enterprise or Open Source, and this is a common enough request that it should probably have a page. High level overview of steps to take is as follows:

1. Set up read-replica workers and coordinator
2. Set up streaming replicas from all primary nodes to all read-replica nodes
3. On primary coordinator, add all read-replica workers as secondary nodes using this. Note the different cluster name- this is important.
http://docs.citusdata.com/en/v8.3/develop/api_udf.html#master-add-secondary-node

Example syntax:

SELECT master_add_secondary_node(
'follower-node-1.local', 5432,
'primary-1.local', 5432,
nodecluster => 'follower-cluster'
);

4. Make sure every primary worker and coordinator has an associated read replica worker added as a secondary!
5. On read replica coordinator, make following setting changes in postgresql.conf:
citus.use_secondary_nodes = 'always';
citus.cluster_name = 'follower-cluster';
Make sure that the read replica coordinator is restarted after changes are made