Add initial docs for PostgreSQL replication

Document how to switch the PostgreSQL primary server to a standby, for
the case where the primary goes down.

Refs #217

doc/postgresql_replication.md

@@ -0,0 +1,14 @@
+# Switch PostgreSQL primary
+
+Change the role in `./nodes/#{new_primary}.json`'s run list:
+
+    -    "role[postgresql_replica]",
+    +    "role[postgresql_primary]",
+
+Delete the `role[postgresql_replica]` role from the old primary's run list.
+
+Run Chef on the new primary, then create the failover file to turn the server into the new primary:
+
+    $ sudo touch /mnt/data/postgresql/failover.trigger
+
+Run Chef on servers that use PostgreSQL to change the IP of the primary server.