Add some docs for node cluster

doc/api/_toc.markdown

@@ -32,6 +32,7 @@
 * [ZLIB](zlib.html)
 * [OS](os.html)
 * [Debugger](debugger.html)
+* [Cluster](cluster.html)
 * Appendixes
   * [Appendix 1: Recommended Third-party Modules](appendix_1.html)
   * [Appendix 2: Deprecated API's](appendix_2.html)

doc/api/cluster.markdown

@@ -0,0 +1,23 @@
+## Cluster
+
+A single instance of Node runs in a single thread. To take advantage of
+multi-core systems the user will sometimes want to launch a cluster of Node
+processes to handle the load.
+
+By starting node with the `cluster` argument, Node will detect the number of
+CPUs on the machine and start that many processes. For example suppose we
+had a simple HTTP server in server.js:
+
+    require('http').createServer(function(req, res) {
+      res.writeHead(200);
+      res.end('hello world\n');
+    }).listen(8000);
+
+If we start it like this
+
+    % node cluster server.js 
+    Detected 2 cpus
+    Worker 2438 online
+    Worker 2437 online
+
+Node will automatically share port 8000 between the multiple instances.