vagrant: add README (#7215)

plugins/vagrant/README.md

@@ -0,0 +1,9 @@
+# Vagrant plugin
+
+This plugin adds completion for [Vagrant](https://www.vagrantup.com/)
+
+To use it, add `vagrant` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... vagrant)
+```