doc: add a section to the FAQ on how to retrigger hwdb changes

I'm typing this way too often into bugreports

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

doc/faqs.dox

@@ -54,4 +54,35 @@ option is not exposed by the intermediary, it cannot be configured by the
 client. Also some configuration options that are provided by the
 intermediary may not be libinput-specific configuration options.
 
+@section faq_hwdb_changes How to apply hwdb changes
+
+Sometimes users are asked to test updates to the <a
+href="https://www.freedesktop.org/software/systemd/man/hwdb.html">udev
+hwdb</a> or patches that include a change to the hwdb.
+
+If you are testing a patch with a hwdb update, the file will be installed
+in the correct location. User-specific (i.e. user-written) hwdb entries for
+testing and debugging must be in `/etc/udev/hwdb.d/99-filename.hwdb`. You
+may rename the `filename` portion to something more useful, but make sure
+you keep the `.hwdb` suffix. Do not modify files or save files in
+`/usr/lib/udev/hwdb.d`.
+
+Figure out the event node name of your device. Run `sudo evemu-describe`
+with no arguments to get a list. If your device is `/dev/input/event4`,
+your event node name is `event4` and you will need to replace that in the
+commands below.
+
+Rebuild the hwdb and load the new set on your device:
+
+    sudo udevadm hwdb --reload
+    sudo udevadm trigger /sys/class/input/eventX
+    sudo udevadm test /sys/class/input/eventX
+
+With the event node matching your device. Make sure the udev property
+appears (or does not appear, whichever applies) in the output of the test
+command.
+
+Once the output matches expectations, restart X or your Wayland
+compositor or reboot.
+
 */