docs: tell AppImage users to re-enable start at login after upgrading

Releases up to v0.6.0 wrote the AppImage path into the autostart entry
unquoted, so an entry made by them never starts when the path contains a
space, while the menu item still shows it as on. Turning the item off and
on rewrites the entry with the quoted path.

README.md

@@ -107,7 +107,10 @@ pick one:
 - `usage-monitor_X.Y.Z_amd64.AppImage`: make it executable
   (`chmod +x`) and run it. Like the Windows exe it is portable and keeps
   `settings.json` next to the AppImage file, so give it a folder you can
-  write to. The AppImage self-updates in place.
+  write to. The AppImage self-updates in place. If you turned on "Start
+  at login" in v0.6.0 or earlier and the AppImage path contains a space,
+  turn it off and on again once: those releases wrote the path unquoted,
+  so the app never started at login.
 - `usage-monitor_X.Y.Z_amd64.deb`: install it with
   `sudo apt install ./usage-monitor_X.Y.Z_amd64.deb`. The app
   still checks for updates, but installing one is left to you: download the