XXMessengerClient

Breaking changes:

• Rename Environment to NDFEnvironment.
• Update NewOrLoadUd functor parameters.

New features:

• Add XXMessengerClient library.
• Update documentation.

Docs/XXMessengerClient.md

@@ -39,6 +39,9 @@ func start(messenger: Messenger) throws {
     try messenger.load()
   }
 
+  // start messenger's network follower:
+  try messenger.start()
+
   // check if messenger is connected:
   if messenger.isConnected() == false {
     // start end-to-end connection: