[Readme] reference screenshots on widgets, add more.

Author: fuzzagrosner

README.md

@@ -78,24 +78,42 @@ Now you replace widgets that are included in this library with their "P" counter
 
 `FlatButton`/`CupertinoButton` -> `PFlatButton`
 
+![Material Flat Button](/screenshots/androidflat.png)
+![Cupertino Flat Button](/screenshots/iosflat.png)
+
 `AppBar`/`CupertinoNavigationBar` -> `PNavigationBar`
 
+![Android Nav](/screenshots/androidnav.png)
+![iOS Nav](/screenshots/iosnav.png)
+
 `SliverAppBar`/`CupertinoSliverNavigationBar` -> `PSliverNavigationBar`
 
 `Slider`/`CupertinoSlider` -> `PSlider`
 
+![Sliders](/screenshots/sliders.png)
+
 `Switch`/`CupertinoSwitch` -> `PSwitch`
 
+![Switch](/screenshots/switches.png)
+
 `BottomNavigationBar`/`CupertinoTabBar` -> `PTabBar`
 
+![Bottom Navigation Android](/screenshots/androidtabs.png)
+![Bottom Navigation iOS](/screenshots/iostabs.png)
+
 `Scaffold`/`CupertinoScaffold` -> `PScaffold`
 
 `CircularProgressIndicator`/`CupertinoActivityIndicator` -> `PActivityIndicator`
 
+![Progress](/screenshots/progress.png)
+
 `BackButton`/`CupertinoNavigationBarBackButton` -> `PBackButton`
 
 `AlertDialog`/`CupertinoAlertDialog` -> `PAlertDialog`
 
+![Android Alert](/screenshots/androidalert.png)
+![Ios Alert](/screenshots/iosalert.png)
+
 ### Properties Specific to a platform have a prefix
 Any widgets that have ios-only or android-only counterparts, they are prefixed to `android`/`ios` accordingly:
 

example/lib/button_page.dart

@@ -40,6 +40,22 @@ class ButtonPage extends StatelessWidget {
                 renderPlatform: TargetPlatform.iOS,
                 onPressed: () {},
               ),
+              PFlatButton(
+                child: Text("Flat Platform Button"),
+                onPressed: () {},
+              ),
+              PTheme.android(
+                PFlatButton(
+                  child: Text("Android Flat Button"),
+                  onPressed: () {},
+                ),
+              ),
+              PTheme.ios(
+                PFlatButton(
+                  child: Text("iOS Flat Button"),
+                  onPressed: () {},
+                ),
+              ),
             ],
           ),
         ],