ant: add README (#7375)

plugins/ant/README.md

+# Ant
+
+This plugin provides completion for [Ant](https://ant.apache.org/).
+
+To use it add ant to the plugins array in your zshrc file.
+
+```bash
+plugins=(... ant)
+```
+
+It caches ant targets in a file named `.ant_targets`, you might want to add that to
+your `.gitignore` file.