add hint regarding btrfs drives for NEXTCLOUD_DATADIR

Signed-off-by: Simon L <szaimen@e.mail.de>

readme.md

@@ -537,7 +537,7 @@ If you already have a backup solution in place, you may want to hide the backup
 
 You can configure the Nextcloud container to use a specific directory on your host as data directory. You can do so by adding the environmental variable `NEXTCLOUD_DATADIR` to the docker run command of the mastercontainer (but before the last line `nextcloud/all-in-one:latest`! If it was started already, you will need to stop the mastercontainer, remove it (no data will be lost) and recreate it using the docker run command that you initially used). Allowed values for that variable are strings that start with `/` and are not equal to `/`. The chosen directory or volume will then be mounted to `/mnt/ncdata` inside the container.
 
-- An example for Linux is `--env NEXTCLOUD_DATADIR="/mnt/ncdata"`.
+- An example for Linux is `--env NEXTCLOUD_DATADIR="/mnt/ncdata"`. ⚠️ Please note: If you should be using and external BTRFS drive that is mounted to `/mnt/ncdata`, make sure to choose a subfolder like e.g. `/mnt/ncdata/nextcloud` as datadir, since the root folder is not suited as datadir in that case. See https://github.com/nextcloud/all-in-one/discussions/2696.
 - On macOS it might be `--env NEXTCLOUD_DATADIR="/var/nextcloud-data"`
 - For Synology it may be `--env NEXTCLOUD_DATADIR="/volume1/docker/nextcloud/data"`. 
 - On Windows it might be `--env NEXTCLOUD_DATADIR="/run/desktop/mnt/host/c/ncdata"`. (This path is equivalent to `C:\ncdata` on your Windows host so you need to translate the path accordingly. Hint: the path that you enter needs to start with `/run/desktop/mnt/host/`. Append to that the exact location on your windows host, e.g. `c/ncdata` which is equivalent to `C:\ncdata`.)