Document the need to update custom modules

doc/migrations/5.0.0_5.1.0.md

@@ -59,3 +59,6 @@ The archive functionality recently introduced has been extended to support many
 ## Removal of deprecated modules
 * `mod_revproxy` - removed from the code base as it was unsupported since 4.2.0.
 * `mod_aws_sns` - its functionality is fully covered by [`mod_event_pusher`](../modules/mod_event_pusher.md).
+
+## Internal module configuration rework
+If you are using your own extension modules (or services), you need to update the code. The most important change is that configuration options were stored in proplists before, and now they are stored in maps, so e.g. the `start/2` function of your module should expect a map as the second argument.