Deprecate "devicemapper" storage driver.

The `devicemapper` storage driver is deprecated in favor of `overlay2`, and will
be removed in a future release. Users of the `devicemapper` storage driver are
recommended to migrate to a different storage driver, such as `overlay2`, which
is now the default storage driver.

The `devicemapper` storage driver facilitates running Docker on older (3.x) kernels
that have no support for other storage drivers (such as overlay2, or AUFS).

Now that support for `overlay2` is added to all supported distros (as they are
either on kernel 4.x, or have support for multiple lowerdirs backported), there
is no reason to continue maintenance of the `devicemapper` storage driver.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/deprecated.md

@@ -19,6 +19,22 @@ The following list of features are deprecated in Engine.
 To learn more about Docker Engine's deprecation policy,
 see [Feature Deprecation Policy](https://docs.docker.com/engine/#feature-deprecation-policy).
 
+### device mapper storage driver
+
+**Deprecated in Release: v18.09.0**
+
+The `devicemapper` storage driver is deprecated in favor of `overlay2`, and will
+be removed in a future release. Users of the `devicemapper` storage driver are
+recommended to migrate to a different storage driver, such as `overlay2`, which
+is now the default storage driver.
+
+The `devicemapper` storage driver facilitates running Docker on older (3.x) kernels
+that have no support for other storage drivers (such as overlay2, or AUFS).
+
+Now that support for `overlay2` is added to all supported distros (as they are
+either on kernel 4.x, or have support for multiple lowerdirs backported), there
+is no reason to continue maintenance of the `devicemapper` storage driver.
+
 ### Reserved namespaces in engine labels
 
 **Deprecated in Release: v18.06.0**