docs(replay): Add migration docs

Author: mydea

packages/replay/MIGRATION.md

@@ -52,3 +52,8 @@ import something from '@sentry/replay/submodule';
 ```
 
 If you only imported from `@sentry/replay`, this will not affect you.
+
+## Session object is now a POJO (https://github.com/getsentry/sentry-javascript/pull/6417)
+
+The `Session` object exported from Replay is now a POJO, instead of a class.
+This should not affect you unless you specifically accessed this class & did custom things with it.