update react perf docs issue 8060 and 6174

Author: dhyey35

docs/docs/addons-perf.md

@@ -14,6 +14,10 @@ next: test-utils.html
 import Perf from 'react-addons-perf' // ES6
 var Perf = require('react-addons-perf') // ES5 with npm
 var Perf = React.addons.Perf; // ES5 with react-with-addons.js
+
+//If Perf is not accessible from console i.e throws RefrenceError: 
+//Perf is not defined, create a global 
+window.Perf = Perf;
 ```
 
 
@@ -32,7 +36,7 @@ See these articles by the [Benchling Engineering Team](http://benchling.engineer
 
 If you're benchmarking or seeing performance problems in your React apps, make sure you're testing with the [minified production build](/react/downloads.html). The development build includes extra warnings that are helpful when building your apps, but it is slower due to the extra bookkeeping it does.
 
-However, the perf tools described on this page only work when using the development build of React. Therefore, the profiler only serves to indicate the _relatively_ expensive parts of your app.
+However, the perf tools described on this page only work when using the development build of React. If you set NODE_ENV to _production_ than it will not work.  Therefore, the profiler only serves to indicate the _relatively_ expensive parts of your app.
 
 ### Using Perf
 
@@ -56,6 +60,33 @@ The following methods use the measurements returned by [`Perf.getLastMeasurement
 
 * * *
 
+## Example Usage:
+
+We will take the comments example from old official tutorial. The example code is availabe on [github](https://github.com/dhyey35/react-example-for-performance/blob/master/public/scripts/example.js).
+
+```javascript
+//Using react-with-addons.js in ES5
+var Perf = React.addons.Perf;
+
+var Comment = React.createClass({
+  rawMarkup: function() {
+    var md = new Remarkable();
+    var rawMarkup = md.render(this.props.children.toString());
+    return { __html: rawMarkup };
+  },
+  //code...
+  
+ReactDOM.render(
+  <CommentBox url='/api/comments' pollInterval={2000} />,
+  document.getElementById('content')
+);
+```
+Open the developer console in the browser and type `Perf.start()` than followed by the action you want to monitor eg: submitting a form. Than type `Perf.stop()` in the console and than type `Perf.getLastMeasurements()` to get the measurements .See the Reference below for more methods.
+
+![](/react/img/docs/addons-perf.png)
+
+A **Live Example** of the above code is available on [react-example-for-performance](https://react-example-for-performance.herokuapp.com/)
+* * *
 ## Reference
 
 ### `start()`