adds docs on how to use with Antennapod

README.md

@@ -76,6 +76,18 @@ These features are all working with Antennapod:
     - Device API
     - Device Synchronization API
 
+To start using with two devices, especially if you want to transfer state
+from old_phone to new_phone:
+
+#. Log in with old_phone and force a full sync.
+#. Log in with new_phone, but select old_phone as the device. Subscriptions will sync.
+#. Log out with new_phone.
+#. Log in with new_phone and create a new device ID for it.
+#. Use the API (such as with curl) to [create a sync group](https://gpoddernet.readthedocs.io/en/latest/api/reference/sync.html#device-synchronization-api) with both devices.
+
+After that, episode state will sync between them, and a new subscription on
+either one will propagate to the other.
+
 ### Development
 
 ```