added demo gif and updated readme for using_bottom_nav_bar and using_tabs example

Author: nisrulz

using_bottom_nav_bar/README.md

@@ -1,8 +1,14 @@
-# using_bottom_nav_bar
+# Using Bottom Navigation Bar
+
+A material widget displayed at the bottom of an app for selecting among a small number of views.
+
+The bottom navigation bar consists of multiple items in the form of labels, icons, or both, laid out on top of a piece of material. It provides quick navigation between the top-level views of an app. For larger screens, side navigation may be a better fit.
+
+Read [[Documentation](https://docs.flutter.io/flutter/material/BottomNavigationBar-class.html)] [[Material Design Spec](https://material.io/guidelines/components/bottom-navigation.html)]
+
+<img src="demo_img.gif" height="600em" />
 
-A new Flutter project.
 
 ## Getting Started
 
-For help getting started with Flutter, view our online
-[documentation](http://flutter.io/).
+For help getting started with Flutter, view online [documentation](http://flutter.io/).

using_bottom_nav_bar/demo_img.gif

@@ -1,8 +1,12 @@
-# using_tabs
+# Using Tab Bar
+
+A material design widget that displays a horizontal row of tabs.
+
+Read [[Documentation](https://docs.flutter.io/flutter/material/TabBar-class.html)] [[Material Design Spec](https://material.io/guidelines/components/tabs.html)]
+
+<img src="demo_img.gif" height="600em" />
 
-A new Flutter project.
 
 ## Getting Started
 
-For help getting started with Flutter, view our online
-[documentation](http://flutter.io/).
+For help getting started with Flutter, view online [documentation](http://flutter.io/).