docs: add information on the KEYBOARD variable.

This documents the solution @Carunga found in sytone#11

README.md

@@ -49,6 +49,7 @@ docker run -d `
 | PGID                 | Set the group ID for the continer user. `911` by default.                                                                                                                                              |
 | TZ                   | Set the Time Zone for the container, should match your TZ. `Etc/UTC` by default. See [List of tz database time zones](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) for valid options. |
 | DOCKER_MODS          | Use to add mods to the container like git. E.g. `DOCKER_MODS=linuxserver/mods:universal-git` See [Docker Mods](https://github.com/linuxserver/docker-mods) for details.                                |
+| KEYBOARD             | Used to se the keyboard being used for input. E.g. `KEYBOARD=en-us-qwerty` or `KEYBOARD=de-de-qwertz` a list of other possible values (not tested) can be found at https://github.com/linuxserver/docker-digikam#keyboard-layouts |
 
 ## Using Docker Compose