More documentation

docs/backends/firebase.md

@@ -1,4 +1,14 @@
 # Google Firebase
+https://firebase.google.com/
+
+## Requirements
+This backend requires a suitable Matrix client capable of performing Firebase authentication and passing the following
+information:
+- Firebase User ID as Matrix username
+- Firebase token as Matrix password
+
+If your client is Riot, you will need a custom version.
+
 ## Configuration
 To be completed. For now, see default structure and values:
 ```
@@ -6,4 +16,4 @@ firebase:
   enabled: false
   credentials: '/path/to/firebase/credentials.json'
   database: 'https://my-project.firebaseio.com/'
-```
+```