docs are good :)

README.md

@@ -36,6 +36,29 @@ Docker Compose installation options.
 
 (Used only for RedHat/CentOS.) You can enable the Edge or Test repo by setting the respective vars to `1`.
 
+### devicemapper
+
+Out of the box this role deploys docker with the overlay driver, but the only
+officially supported configuration of Docker on RedHat/CentOS is to use device
+mapper as the storage driver (Not to be confused with `docker volume`). Docker
+can configure the lvm thin provisioning when provided a raw block device (such
+as a second disk). To deploy this configuration, set the following variables in
+your deploy:
+
+    docker_devicemapper_raw_device: /dev/sdb # Or /dev/vdb or ...
+    docker_configure_daemon: true
+
+For more information, see the [device mapper driver][devicemapper] documentation.
+
+### Docker daemon configuration
+
+To configure devicemapper as the storage driver, this role has to configure the
+docker daemon via `daemon.json`, but you can also use this to configure the
+daemon as you wish by overriding `docker_daemon_config`. This yaml hash is then
+converted to json. Making changes to `daemon.json` then require a restart of the
+service, but this role does not default to restarting the service. To trigger a
+restart when reconfiguring the daemon, set `docker_restart` to `true`.
+
 ## Use with Ansible (and `docker` Python library)
 
 Many users of this role wish to also use Ansible to then _build_ Docker images and manage Docker containers on the server where Docker is installed. In this case, you can easily add in the `docker` Python library using the `geerlingguy.pip` role:
@@ -71,3 +94,5 @@ MIT / BSD
 ## Author Information
 
 This role was created in 2017 by [Jeff Geerling](https://www.jeffgeerling.com/), author of [Ansible for DevOps](https://www.ansiblefordevops.com/).
+
+[devicemapper]: https://docs.docker.com/engine/userguide/storagedriver/device-mapper-driver/