Add warning about SQLite to config

mautrix_signal/example-config.yaml

@@ -34,6 +34,7 @@ appservice:
     max_body_size: 1
 
     # The full URI to the database. SQLite and Postgres are supported.
+    # However, SQLite support is extremely experimental and should not be used.
     # Format examples:
     #   SQLite:   sqlite:///filename.db
     #   Postgres: postgres://username:password@hostname/dbname