docs: Add PubNub as a networking option (#4007)

Add PubNub as a networking option.

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Co-authored-by: Lukas Klingsbo <lukas.klingsbo@gmail.com>

Author: ByteJoseph

doc/README.md

@@ -187,3 +187,5 @@ If you are building a multiplayer game, here are some recommendations of package
 - [Firebase](https://firebase.google.com/): Provides dozens of services that can be used to write
 simpler multiplayer experiences.
 - [Supabase](https://supabase.com/): A cheaper alternative to Firebase, based on Postgres.
+- [PubNub](https://github.com/pubnub/dart): A real-time messaging network for syncing game lobbies
+and player updates.