added link to vmwareworkstation driver (docker#3542)

* added link to vmwareworkstation driver

Signed-off-by: Victoria Bialas <[email protected]>

* added unofficial, unsupported

Signed-off-by: Victoria Bialas <[email protected]>

Author: londoncalling

machine/drivers/index.md

@@ -17,4 +17,5 @@ title: Drivers
 -   [Oracle VirtualBox](virtualbox.md)
 -   [VMware vCloud Air](vm-cloud.md)
 -   [VMware Fusion](vm-fusion.md)
--   [VMware vSphere](vsphere.md)
+-   [VMware vSphere](vsphere.md)
+-   [VMware Workstation](https://github.com/pecigonzalo/docker-machine-vmwareworkstation) (unofficial plugin, not supported by Docker)