Add the DRBD Docker Volume Plugin to the documentation

Signed-off-by: Roland Kammerer <roland.kammerer@linbit.com>

docs/extend/plugins.md

@@ -52,6 +52,7 @@ Plugin
 [Blockbridge plugin](https://github.com/blockbridge/blockbridge-docker-volume)      | A volume plugin that provides access to an extensible set of container-based persistent storage options. It supports single and multi-host Docker environments with features that include tenant isolation, automated provisioning, encryption, secure deletion, snapshots and QoS.
 [Contiv Volume Plugin](https://github.com/contiv/volplugin)                         | An open source volume plugin that provides multi-tenant, persistent, distributed storage with intent based consumption using ceph underneath.
 [Convoy plugin](https://github.com/rancher/convoy)                                  | A volume plugin for a variety of storage back-ends including device mapper and NFS. It's a simple standalone executable written in Go and provides the framework to support vendor-specific extensions such as snapshots, backups and restore.
+[DRBD plugin](https://www.drbd.org/en/supported-projects/docker)                    | A volume plugin that provides highly available storage replicated by [DRBD](https://www.drbd.org). Data written to the docker volume is replicated in a cluster of DRBD nodes.
 [Flocker plugin](https://clusterhq.com/docker-plugin/)                              | A volume plugin that provides multi-host portable volumes for Docker, enabling you to run databases and other stateful containers and move them around across a cluster of machines.
 [gce-docker plugin](https://github.com/mcuadros/gce-docker)                         | A volume plugin able to attach, format and mount Google Compute [persistent-disks](https://cloud.google.com/compute/docs/disks/persistent-disks).
 [GlusterFS plugin](https://github.com/calavera/docker-volume-glusterfs)             | A volume plugin that provides multi-host volumes management for Docker using GlusterFS.