docs: Transparent window

docs/api/frameless-window.md

@@ -13,6 +13,31 @@ var BrowserWindow = require('browser-window');
 var win = new BrowserWindow({ width: 800, height: 600, frame: false });
 ```
 
+## Transparent window
+
+By setting the `transparent` option to `true`, you can also make the frameless
+window transparent:
+
+```javascript
+var win = new BrowserWindow({ transparent: true, frame: false });
+```
+
+### Limitations
+
+* You can not click through the transparent area, we are going to introduce an
+  API to set window shape to solve this, but currently blocked at an
+  [upstream bug](https://code.google.com/p/chromium/issues/detail?id=387234).
+* Transparent window is not resizable, setting `resizable` to `true` may make
+  transprent window stop working on some platforms.
+* The `blur` filter only applies to the web page, so there is no way to apply
+  blur effect to the content bellow the window.
+* On Windows transparent window will not work when DWM is disabled.
+* On Linux users have to put `--enable-transparent-visuals --disable-gpu` in
+  command line to disable GPU and allow ARGB to make transparent window, this is
+  caused by an upstream bug that [alpha channel doesn't work on some NVidia
+  drivers](https://code.google.com/p/chromium/issues/detail?id=369209) on Linux.
+* On Mac the native window shadow will not show for transparent window.
+
 ## Draggable region
 
 By default, the frameless window is non-draggable. Apps need to specify