[README] Add Xamarin.Forms banner (#757)

This banner is similar to what we're using at open.xamarin.com
and to the installer's new splash screen images.

Xamarin.iOS is already using its banner.

README.md

@@ -1,3 +1,4 @@
+<img src="banner.png" alt="Xamarin.Forms banner" height="145" >
 # Xamarin.Forms #
 
 Xamarin.Forms provides a way to quickly build native apps for iOS, Android, and Windows, completely in C#.