added more comments to using_tabs example

using_tabs/lib/FirstTab.dart

@@ -6,6 +6,7 @@ class First extends StatelessWidget {
     return new Container(
       child: new Center(
         child: new Column(
+          // center the children
           mainAxisAlignment: MainAxisAlignment.center,
           children: <Widget>[
             new Icon(

using_tabs/lib/SecondTab.dart

@@ -6,6 +6,7 @@ class Second extends StatelessWidget {
     return new Container(
       child: new Center(
         child: new Column(
+          // center the children
           mainAxisAlignment: MainAxisAlignment.center,
           children: <Widget>[
             new Icon(

using_tabs/lib/ThirdTab.dart

@@ -6,6 +6,7 @@ class Third extends StatelessWidget {
     return new Container(
       child: new Center(
         child: new Column(
+          // center the children
           mainAxisAlignment: MainAxisAlignment.center,
           children: <Widget>[
             new Icon(

using_tabs/lib/main.dart

@@ -17,18 +17,22 @@ class MyHome extends StatefulWidget {
   MyHomeState createState() => new MyHomeState();
 }
 
+// SingleTickerProviderStateMixin is used for animation
 class MyHomeState extends State<MyHome> with SingleTickerProviderStateMixin {
+  // Create a tab controller
   TabController controller;
 
   @override
   void initState() {
     super.initState();
 
+    // Initialize the Tab Controller
     controller = new TabController(length: 3, vsync: this);
   }
 
   @override
   void dispose() {
+    // Dispose of the Tab Controller
     controller.dispose();
     super.dispose();
   }
@@ -40,10 +44,13 @@ class MyHomeState extends State<MyHome> with SingleTickerProviderStateMixin {
         appBar: new AppBar(
           // Title
           title: new Text("Using Tabs"),
+          // Set the background color of the App Bar
           backgroundColor: Colors.blue,
+          // Set the bottom property of the Appbar to include a Tab Bar
           bottom: new TabBar(
             tabs: <Tab>[
               new Tab(
+                // set icon to the tab
                 icon: new Icon(Icons.favorite),
               ),
               new Tab(
@@ -53,16 +60,19 @@ class MyHomeState extends State<MyHome> with SingleTickerProviderStateMixin {
                 icon: new Icon(Icons.airport_shuttle),
               ),
             ],
+            // setup the controller
             controller: controller,
           ),
         ),
-        // Body
+        // Set the TabBar view as the body of the Scaffold
         body: new TabBarView(
+          // Add tabs as widgets
           children: <Widget>[
             new first_tab.First(),
             new second_tab.Second(),
             new third_tab.Third()
           ],
+          // set the controller
           controller: controller,
         ));
   }