helm: add README (#7325)

plugins/helm/README.md

@@ -0,0 +1,9 @@
+# Helm plugin
+
+This plugin adds completion for [Helm](https://helm.sh/), the Kubernetes package manager.
+
+To use it, add `helm` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... helm)
+```