docs: Add an example

lib/omemo_dart.dart

@@ -10,6 +10,7 @@ export 'src/omemo/encrypted_key.dart';
 export 'src/omemo/encryption_result.dart';
 export 'src/omemo/events.dart';
 export 'src/omemo/fingerprint.dart';
+export 'src/omemo/ratchet_map_key.dart';
 export 'src/omemo/sessionmanager.dart';
 export 'src/trust/base.dart';
 export 'src/trust/btbv.dart';

lib/src/omemo/ratchet_map_key.dart

@@ -1,6 +1,5 @@
 import 'package:meta/meta.dart';
 
-@internal
 @immutable
 class RatchetMapKey {
 

lib/src/omemo/sessionmanager.dart

@@ -78,6 +78,7 @@ class OmemoSessionManager {
 
   /// The trust manager
   final TrustManager _trustManager;
+  TrustManager get trustManager => _trustManager;
   
   /// A stream that receives events regarding the session
   Stream<OmemoEvent> get eventStream => _eventStreamController.stream;