ppapi: Add pepper flash tutorial

Fix https://github.com/atom/electron/issues/1674

docs/tutorial/using-pepper-flash-plugin.md

@@ -0,0 +1,55 @@
+# Using Pepper Flash Plugin
+
+Pepper flash plugin is now supported. To use pepper flash plugin in Electron, you should manually specify the location of pepper flash plugin and then enable it in your application.
+
+## Prepare a copy of flash plugin
+
+On OS X and Linux, the detail of pepper flash plugin can be found by navigating `chrome://plugins` in Chrome browser. Its location and version are useful for electron's pepper flash support. You can also copy it to anywhere else.
+
+## Add Electron switch
+
+You can directly add `--ppapi-flash-path` and `ppapi-flash-version` to electron commandline or by `app.commandLine.appendSwitch` method before app ready event. Also, add the `plugins` switch of `browser-window`. For example,
+
+```javascript
+var app = require('app');
+var BrowserWindow = require('browser-window');
+
+// Report crashes to our server.
+require('crash-reporter').start();
+
+// Keep a global reference of the window object, if you don't, the window will
+// be closed automatically when the javascript object is GCed.
+var mainWindow = null;
+
+// Quit when all windows are closed.
+app.on('window-all-closed', function() {
+  if (process.platform != 'darwin')
+    app.quit();
+});
+
+// Specify flash path.
+// On Windows, it might be /path/to/pepflashplayer.dll
+// On Mac, /path/to/PepperFlashPlayer.plugin
+// On Linux, /path/to/libpepflashplayer.so
+app.commandLine.appendSwitch('ppapi-flash-path', '/path/to/libpepflashplayer.so');
+
+// Specify flash version, for example, v17.0.0.169
+app.commandLine.appendSwitch('ppapi-flash-version', '17.0.0.169');
+
+app.on('ready', function() {
+  mainWindow = new BrowserWindow({
+    'width': 800,
+    'height': 600,
+    'web-preferences':
+      'plugins': true
+  });
+  mainWindow.loadUrl('file://' + __dirname + '/index.html');
+  // Something else
+});
+```
+
+## Enable flash plugin in a `<webview>` tag
+Add `plugins` attribute to `<webview>` tag.
+```html
+<webview src="http://www.adobe.com/software/flash/about/" plugins></webview>
+```