Merge pull request #17241 from gondor/docs-extend-plugins

docs: Add Netshare volume plugin to extend/plugins

docs/extend/plugins.md

@@ -57,6 +57,9 @@ The following plugins exist:
   a plugin that provides credentials and secret management using Keywhiz as
   a central repository.
 
+* The [Netshare plugin](https://github.com/gondor/docker-volume-netshare) is a volume plugin
+  that provides volume management for NFS 3/4, AWS EFS and CIFS file systems.
+
 * The [Pachyderm PFS plugin](https://github.com/pachyderm/pachyderm/tree/master/src/cmd/pfs-volume-driver)
   is a volume plugin written in Go that provides functionality to mount Pachyderm File System (PFS)
   repositories at specific commits as volumes within Docker containers.