doctl: add README (#7346)

plugins/doctl/README.md

@@ -0,0 +1,9 @@
+# Doctl
+
+This plugin provides completion for [Doctl](https://github.com/digitalocean/doctl).
+
+To use it add doctl to the plugins array in your zshrc file.
+
+```bash
+plugins=(... doctl)
+```