update doc

Author: fenio

doc/USAGE.md

@@ -6,17 +6,30 @@ The following assumes you have the plugin installed via
 kubectl krew install pv-mounter
 ```
 
-### Scan images in your current kubecontext
+### Mount PVC to local directory
 
 ```shell
-kubectl pv-mounter
+kubectl pv-mounter mount some-ns some-pvc some-mountpoint 
 ```
 
-### Scan images in another kubecontext
+### Unmount / clean stuff
 
 ```shell
-kubectl pv-mounter --context=context-name
+kubectl pv-mounter clean some-ns some-pvc some-mountpoint
 ```
 
 ## How it works
-Write a brief description of your plugin here.
+
+It performs a few tasks. In the case of volumes with RWX (ReadWriteMany) access mode or unmounted RWO (ReadWriteOnce):
+
+* Spawns a POD with a minimalistic image that contains an SSH daemon and binds it to the existing PVC.
+* Creates a port-forward to make it locally accessible.
+* Mounts the volume locally using SSHFS.
+
+For already mounted RWO volumes, it's a bit more complex:
+
+* Spawns a POD with a minimalistic image that contains an SSH daemon and acts as a proxy to an ephemeral container.
+* Creates an ephemeral container within the POD that currently mounts the volume.
+* From that ephemeral container, establishes a reverse SSH tunnel to the proxy POD.
+* Creates a port-forward to the proxy POD onto the port exposed by the tunnel to make it locally accessible.
+* Mounts the volume locally using SSHFS.