Document plugins

Add plugins documentation to root.
Mention configuring proxy plugins and runtime plugins.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

README.md

@@ -184,6 +184,28 @@ defer task.Delete(context)
 err := task.Start(context)
 ```
 
+### Snapshot Plugins
+
+In addition to the built-in Snapshot plugins in containerd, additional external
+plugins can be configured using GRPC. An external plugin is made available using
+the configured name and appears as a plugin alongside the built-in ones.
+
+To add an external snapshot plugin, add the plugin to containerd's config file
+(by default at `/etc/containerd/config.toml`). The string following
+`proxy_plugin.` will be used as the name of the snapshotter and the address
+should refer to a socket with a GRPC listener serving containerd's Snapshot
+GRPC API. Remember to restart containerd for any configuration changes to take
+effect.
+
+```
+[proxy_plugins]
+  [proxy_plugins.customsnapshot]
+    type = "snapshot"
+    address =  "/var/run/mysnapshotter.sock"
+```
+
+See [PLUGINS.md](PLUGINS.md) for how to create plugins
+
 ### Releases and API Stability
 
 Please see [RELEASES.md](RELEASES.md) for details on versioning and stability