[AutoUpdater] add documentation

docs/References/AutoUpdater.md

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+# AutoUpdater {: .doctitle}
+---
+
+[TOC]
+
+`AutoUpdater` Enable apps to automatically update themselves.
+
+
+The `AutoUpdater` module provides an interface for the
+[Squirrel](https://github.com/Squirrel) framework.
+
+You can quickly launch a multi-platform release server for distributing your
+application by using one of these projects:
+
+- [nuts][nuts]: *A smart release server for your applications, using GitHub as a backend. Auto-updates with Squirrel (Mac & Windows)*
+- [squirrel-updates-server][squirrel-updates-server]: *A simple node.js server
+  for Squirrel.Mac and Squirrel.Windows which uses GitHub releases*
+- [squirrel-release-server][squirrel-release-server]: *A simple PHP application for Squirrel.Windows which reads updates from a folder. Supports delta updates.*
+
+## Synopsis
+```javascript
+// listen to error events
+nw.AutoUpdater.on('error', function(msg){
+  console.log(msg);
+});
+
+// listen to update-downloaded event
+nw.AutoUpdater.on('update-downloaded', function(releaseNotes, releaseName, releaseDate, updateURL){
+  console.log(releaseNotes);
+  console.log(releaseName);
+  console.log(releaseDate);
+  console.log(updateURL);
+  nw.AutoUpdater.QuitAndInstall();
+});
+
+// set the update feed url
+nw.AutoUpdater.SetFeedURL('file:///tmp/nwjs.autoupdate.json');
+// the real autoupdate
+nw.AutoUpdater.CheckForUpdates();
+```
+
+## Platform notices
+
+Though `AutoUpdater` provides a uniform API for different platforms, there are
+still some subtle differences on each platform.
+
+### macOS
+
+On macOS, the `AutoUpdater` module is built upon [Squirrel.Mac][squirrel-mac],
+meaning you don't need any special setup to make it work. For server-side
+requirements, you can read [Server Support][server-support]. Note that [App
+Transport Security](https://developer.apple.com/library/content/documentation/General/Reference/InfoPlistKeyReference/Articles/CocoaKeys.html#//apple_ref/doc/uid/TP40009251-SW35) (ATS) applies to all requests made as part of the
+update process. Apps that need to disable ATS can add the
+`NSAllowsArbitraryLoads` key to their app's plist.
+
+**Note:** Your application must be signed for automatic updates on macOS.
+This is a requirement of `Squirrel.Mac`.
+
+We have a [test suite](../../test/manual/autoupdate) to quickly test if the autoupdate feature is working.
+Run `autoupdate.sh` from the folder, it needs 2 parameters:
+* `IDENTITY` is your developer id, to sign the app
+* `DIRECTORY` is your nwjs.app location
+
+### Windows
+
+On Windows, you have to install your app into a user's machine before you can
+use the `AutoUpdater`, so it is recommended that you use the
+[electron-winstaller][installer-lib], [electron-builder][electron-builder-lib] or the [grunt-electron-installer][installer] package to generate a Windows installer.
+
+When using [electron-winstaller][installer-lib] or [electron-builder][electron-builder-lib] make sure you do not try to update your app [the first time it runs](https://github.com/electron/windows-installer#handling-squirrel-events) (Also see [this issue for more info](https://github.com/electron/electron/issues/7155)). It's also recommended to use [electron-squirrel-startup](https://github.com/mongodb-js/electron-squirrel-startup) to get desktop shortcuts for your app.
+
+The installer generated with Squirrel will create a shortcut icon with an
+[Application User Model ID][app-user-model-id] in the format of
+`com.squirrel.PACKAGE_ID.YOUR_EXE_WITHOUT_DOT_EXE`, examples are
+`com.squirrel.slack.Slack` and `com.squirrel.code.Code`. You have to use the
+same ID for your app with `app.setAppUserModelId` API, otherwise Windows will
+not be able to pin your app properly in task bar.
+
+The server-side setup is also different from macOS. You can read the documents of
+[Squirrel.Windows][squirrel-windows] to get more details.
+
+### Linux
+
+There is no built-in support for auto-updater on Linux, so it is recommended to
+use the distribution's package manager to update your app.
+
+## AutoUpdater.SetFeedURL(url)
+
+* `url` String
+
+Sets the `url` and initialize the auto updater.
+
+## AutoUpdater.GetFeedURL()
+
+Returns `String` - The current update feed URL.
+
+## AutoUpdater.CheckForUpdates()
+
+Asks the server whether there is an update. You must call `setFeedURL` before
+using this API.
+
+## AutoUpdater.QuitAndInstall()
+
+Restarts the app and installs the update after it has been downloaded. It
+should only be called after `update-downloaded` has been emitted.
+
+## Event: error (msg)
+
+Emitted when there is an error while updating.
+
+## Event: checking-for-update
+
+Emitted when checking if an update has started.
+
+## Event: update-available
+
+Emitted when there is an available update. The update is downloaded
+automatically.
+
+## Event: update-not-available
+
+Emitted when there is no available update.
+
+## Event: update-downloaded (releaseNotes, releaseName, releaseDate, updateURL)
+
+Emitted when an update has been downloaded, with following parameters:
+
+* `releaseNotes` String
+* `releaseName` String, macOS only, NOT available on windows
+* `releaseDate` Date
+* `updateURL` String
+
+[squirrel-mac]: https://github.com/Squirrel/Squirrel.Mac
+[server-support]: https://github.com/Squirrel/Squirrel.Mac#server-support
+[squirrel-windows]: https://github.com/Squirrel/Squirrel.Windows
+[installer]: https://github.com/electron/grunt-electron-installer
+[installer-lib]: https://github.com/electron/windows-installer
+[electron-builder-lib]: https://github.com/electron-userland/electron-builder
+[app-user-model-id]: https://msdn.microsoft.com/en-us/library/windows/desktop/dd378459(v=vs.85).aspx
+[squirrel-updates-server]: https://github.com/Aluxian/squirrel-updates-server
+[nuts]: https://github.com/GitbookIO/nuts
+[squirrel-release-server]: https://github.com/Arcath/squirrel-release-server