@uppy/xhr-upload: add 'upload-stalled' event (#43)

docs/uppy-core.mdx

@@ -1118,7 +1118,24 @@ uppy.on('upload-retry', (fileID) => {
   console.log('upload retried:', fileID)
 })
 ```
-  
+
+### `upload-stalled`
+
+Fired when an upload has not received any progress in some time (in `@uppy/xhr-upload`, the delay is defined by the `timeout` option). Use this event to display a message on the UI to tell the user they might want to retry the upload.
+
+```js
+uppy.on('upload-stalled', (error, files) => {
+  console.log('upload seems stalled', error, files)
+  const noLongerStalledEventHandler = (file) => {
+    if (files.includes(file)) {
+      console.log('upload is no longer stalled')
+      uppy.off('upload-progress', noLongerStalledEventHandler)
+    }
+  }
+  uppy.on('upload-progress', noLongerStalledEventHandler)
+})
+```
+
 #### `retry-all`
 
 Fired when all failed uploads are retried