added some help on debugging device support in development.md

readme/development.md

@@ -123,6 +123,41 @@ just need to be commited.
 
 Communication to the service always happens via `key-mapper-control`
 
+## Unsupported Devices
+
+Either open up an issue or debug it yourself and make a pull request.
+
+You will need to work with the devices capabilities. You can get those using
+
+```
+sudo evtest
+```
+
+**It tries or doesn't try to map ABS_X/ABS_Y**
+
+Is the device a gamepad? Does the GUI show joystick configurations?
+- if yes, no: adjust `is_gamepad` to loosen up the constraints
+- if no, yes: adjust `is_gamepad` to tighten up the constraints
+Try to do it in such a way that other devices won't break. Also see 
+readme/capabilities.md
+
+**It won't offer mapping a button**
+
+Modify `should_map_event_as_btn`
+
+**The cursor won't move anymore**
+
+Can be difficult. Depending on capabilities the display server might not
+treat events as cursor movements anymore. e.g. mice with EV_ABS capabilities
+won't move the cursor. Or key-mapper removed the EV_ABS capabilities.
+Or due to weird stuff a new capability appears out of nowhere (ABS_VOLUME).
+
+At some point this won't be a problem anymore when key-mapper creates a new
+device for all injected keys for non-keyboards, as well as for generated
+EV_REL events for gamepads.
+
+Modify `_modify_capabilities` to get it to work.
+
 ## Resources
 
 - [Guidelines for device capabilities](https://www.kernel.org/doc/Documentation/input/event-codes.txt)