setOffscreenRender and api docs added

docs/api/browser-window.md

@@ -74,6 +74,8 @@ You can also create a window without chrome by using
     Linux.
   * `standard-window` Boolean - Uses the OS X's standard window instead of the
     textured window. Defaults to `true`.
+  * `offscreen-render` Boolean - The frame of the window will be accessible
+    through the `frame-rendered` event in a buffer. Defaults to `false`.
   * `web-preferences` Object - Settings of web page's features
     * `javascript` Boolean
     * `web-security` Boolean - When setting `false`, it will disable the same-origin
@@ -226,6 +228,15 @@ Emitted when devtools is closed.
 
 Emitted when devtools is focused / opened.
 
+### Event: 'frame-rendered'
+
+* `event` Event
+* `frame` Buffer
+* `size` Number
+
+Emitted when *offscreen render* is enabled, the current frame's pixel data
+and size are available.
+
 ### Event: 'app-command':
 
 Emitted when an [App Command](https://msdn.microsoft.com/en-us/library/windows/desktop/ms646275(v=vs.85).aspx) is invoked. These are typically related to keyboard media keys or browser commands, as well as the "Back" button built into some mice on Windows.
@@ -721,6 +732,11 @@ Returns whether the window is visible on all workspaces.
 
 **Note:** This API always returns false on Windows.
 
+### BrowserWindow.setOffscreenRender(isOffscreen)
+
+Sets the offscreen rendering, if `true` the `frame-rendered` event will fire,
+when the frame changes.
+
 ## Class: WebContents
 
 A `WebContents` is responsible for rendering and controlling a web page.