cluster: add example for message passing

doc/api/cluster.markdown

@@ -34,6 +34,44 @@ Running node will now share port 8000 between the workers:
     Worker 2438 online
     Worker 2437 online
 
+The difference between `cluster.fork()` and `child_process.fork()` is simply
+that cluster allows TCP servers to be shared between workers. `cluster.fork`
+is implemented on top of `child_process.fork`. The message passing API that
+is available with `child_process.fork` is available with `cluster` as well.
+As an example, here is a cluster which keeps count of the number of requests
+in the master process via message passing:
+
+    var cluster = require('cluster');
+    var http = require('http');
+    var numReqs = 0;
+
+    if (cluster.isMaster) {
+      // Fork workers.
+      for (var i = 0; i < 2; i++) {
+        var worker = cluster.fork();
+
+        worker.on('message', function(msg) {
+          if (msg.cmd && msg.cmd == 'notifyRequest') {
+            numReqs++;
+          }
+        });
+      }
+
+      setInterval(function() {
+        console.log("numReqs =", numReqs);
+      }, 1000);
+    } else {
+      // Worker processes have a http server.
+      http.Server(function(req, res) {
+        res.writeHead(200);
+        res.end("hello world\n");
+        // Send message to master process
+        process.send({ cmd: 'notifyRequest' });
+      }).listen(8000);
+    }
+
+
+
 ### cluster.fork()
 
 Spawn a new worker process. This can only be called from the master process.