Add connection_limit sample config

mautrix_signal/example-config.yaml

@@ -16,6 +16,8 @@ homeserver:
     status_endpoint: null
     # Endpoint for reporting per-message status.
     message_send_checkpoint_endpoint: null
+    # Maximum number of simultaneous HTTP connections to the homeserver.
+    connection_limit: 100
 
 # Application service host/registration related details
 # Changing these values requires regeneration of the registration.