Add links to realtime databases (#5279)

src/data/roadmaps/backend/content/107-nosql-databases/103-realtime-databases.md

@@ -1,3 +1,6 @@
 # Realtime databases
 
 A real-time database is broadly defined as a data store designed to collect, process, and/or enrich an incoming series of data points (i.e., a data stream) in real time, typically immediately after the data is created.
+
+[Firebase](https://firebase.google.com/)
+[RethinkDB](https://rethinkdb.com/)