title: Provide a background image to the Toolbar in navigation page
body: <p>I am new to Xamarin.forms, I am unable to provide a background image to the navigation tool bar.
  I am able to set the Title and add a right button on the toolbar using ToolbarItems.Add(...).
  My code snippet is as below</p>
  
  <p>public class App : Application</p>
  
  <p>{</p>
  
  <p>public App ()</p>
  
  <p>{</p>
  
  <p>MainPage = new NavigationPage(new Home());</p>
  
  <p>}</p>
  
  <p>}</p>
  
  <p>public class Home : ContentPage
  {
  public Home ()</p>
  
  <p>{</p>
  
  <p>this.Title = "MyHeader";</p>
  
  <p>ToolbarItems.Add(new ToolbarItem{...});</p>
  
  <p>}</p>
  
  <p>}</p>
  
  <p>Please suggest the ways to provide the background image on the nav tool bar.</p>
  
  <p>Thanks</p>