Update helm-purge.yaml

Document prereq to install https://github.com/robertsmieja/kubectl-purge in order for kubectl purge

Otherwise fails with 
> $ kubectl purge
> error: unknown command "purge" for "kubectl"

plugins/helm-purge.yaml

@@ -1,6 +1,7 @@
 # $HOME/.k9s/plugin.yml
 plugins:
   # Issues a helm delete --purge for the resource associated with the selected pod
+  # Requires https://github.com/robertsmieja/kubectl-purge 
   helm-purge:
     shortCut: Ctrl-P
     description: Helm Purge