app: add will-download event to defaultSession

docs/api/app.md

@@ -330,3 +330,35 @@ Sets the application's [dock menu][dock-menu].
 
 [dock-menu]:https://developer.apple.com/library/mac/documentation/Carbon/Conceptual/customizing_docktile/concepts/dockconcepts.html#//apple_ref/doc/uid/TP30000986-CH2-TPXREF103
 [tasks]:http://msdn.microsoft.com/en-us/library/windows/desktop/dd378460(v=vs.85).aspx#tasks
+
+## `app.defaultSession`
+
+The default session of the app available once the (ready)[app.md#event-ready] event is fired.
+
+```javascript
+app.on('ready', function() {
+  var defaultSession = app.defaultSession;
+})
+```
+
+### Event: 'will-download'
+
+* `event` Event
+* `downloadItem` Object
+  * `url` String
+  * `filename` String
+  * `mimeType` String
+  * `hasUserGesture` Boolean
+* `webContents` (WebContents)[web-contents.md]
+
+Fired when a download is about to start. Calling `preventDefault()`
+will cancel the download.
+
+```javascript
+app.defaultSession.on('will-download', function(e, downloadItem, webContents) {
+  e.preventDefault();
+  require('request')(downloadItem.url, function(data) {
+    require('fs').writeFileSync('/somewhere', data);
+  });
+});
+```