Add the Local Persist plugin

I wasn't 100% sure if it was appropriate to add plugin here, but @thaJeztah invited me to do so, so here it is!

(see https://github.com/CWSpear/local-persist/issues/17#issuecomment-188523784)

Signed-off-by: Cameron Spear <cameronspear@gmail.com>

docs/extend/plugins.md

@@ -99,6 +99,11 @@ The following plugins exist:
   Neutron, the OpenStack networking service. It includes an IPAM
   driver as well.
 
+* The [Local Persist Plugin](https://github.com/CWSpear/local-persist) 
+  extends the default `local` driver's functionality by allowing you specify 
+  a mountpoint anywhere on the host, which enables the files to *always persist*, 
+  even if the volume is removed via `docker volume rm`.
+
 ## Troubleshooting a plugin
 
 If you are having problems with Docker after loading a plugin, ask the authors