added example on pattern tokens usage

examples/patternLayout-tokens.js

@@ -0,0 +1,21 @@
+var log4js = require("log4js");
+
+var config = {
+    "appenders": [
+      {
+        "type": "console",
+        "layout": {
+          "type": "pattern",
+          "pattern": "%[%r (%x{pid}) %p %c -%] %m%n",
+          "tokens": {
+            "pid" : function() { return process.pid; }
+          }
+        }
+      }
+    ]
+  };
+
+log4js.configure(config, {});
+
+var logger = log4js.getLogger("app");
+logger.info("Test log message");