Add documentation about the REPL

README.md

@@ -89,3 +89,31 @@ The `exec`, `env`, `argv`, and `silent` configs are passed to the
   the parent.  Called in the context of the worker, so you can reply by
   looking at `this`.
 
+## REPL
+
+Cluster-master provides a REPL into the master process so you can inspect
+the state of your cluster. By default the REPL is accessible by a socket 
+written to the root of the directory, but you can override it with the 
+`CLUSTER_MASTER_REPL` environment variable. You can access the REPL with 
+[socat](http://www.dest-unreach.org/socat/) like so:
+
+```
+socat ./cluster-master-socket stdin
+```
+
+The REPL provides you with access to these objects or functions: 
+
+* `repl` 
+* `resize(n)` 
+* `restart(cb)`
+* `quit()`
+* `cluster`
+* `size`
+* `connections`
+* `workers`
+* `select(pid)`
+* `pids`
+* `ages`
+* `states`
+* `debug`
+* `sock`