Added AV.Cloud.onLogin doc

Author: killme2008

lib/cloud.js

@@ -194,6 +194,19 @@
     */
    AV.Cloud.afterDelete = function(name, func){};
 
+   /**
+    * Registers a on-login function.
+    * <br/><strong>Available in Cloud Code only</strong>.
+    * If you want to use onLogin for an user in the AV JavaScript SDK, you should pass the function:
+    * <code><pre>
+  AV.Cloud.onLogin(function(request, response) {
+    // code here
+  });
+    * </pre></code>
+    * @param {Function} func The function to run before an user signin. This function should take two parameters a AV.Cloud.FunctionRequest and a AV.Cloud.FunctionResponse.
+    */
+   AV.Cloud.onLogin = function(func) {};
+
    /**
     * Makes an HTTP Request.
     * <br/><strong>Available in Cloud Code only</strong>.