docs: add section for how to handle update servers with auth (#23000)

In some situations, we can't use auto update to communicate with our update server directly, and proxies can be tricky to get working. I spent a loooong time exploring a lot of options before coming across the comment I linked, so It thought it could be beneficial to people like me of the future.

docs/tutorial/updates.md

@@ -132,6 +132,10 @@ autoUpdater.on('error', message => {
 })
 ```
 
+## Handing Updates Manually
+
+Because the requests made by Auto Update aren't under your direct control, you may find situations that are difficult to handle (such as if the update server is behind authentication). The `url` field does support files, which means that with some effort, you can sidestep the server-communication aspect of the process. [Here's an example of how this could work](https://github.com/electron/electron/issues/5020#issuecomment-477636990).
+
 [now]: https://zeit.co/now
 [hazel]: https://github.com/zeit/hazel
 [nuts]: https://github.com/GitbookIO/nuts