added "good" private property example

Author: Nicholas Kinsey

README.md

@@ -900,6 +900,9 @@
     // bad
     this.__firstName__ = 'Panda';
     this.firstName_ = 'Panda';
+    
+    // good
+    this._firstName = 'Panda';
     ```
 
   - Name your functions. This is helpful for stack traces.