Add note to README about battery powered devices.

README.md

@@ -37,7 +37,13 @@ easier to set up using that as an alternative.
 
 Note that devices sometimes get firmware upgrades, or incompatible
 versions are sold under the same model name, so it is possible that
-the device will not work despite being listed.
+the device will not work despite being listed. 
+
+Battery powered devices such as door and window sensors, smoke alarms
+etc which do not use a hub will be impossible to support locally, due
+to the power management that they need to do to get acceptable battery
+life.  Currently hubs are also unsupported, but this is being worked
+on.
 
 A list of currently supported devices can be found in the [DEVICES.md](https://github.com/make-all/tuya-local/blob/main/DEVICES.md) file.