celery: add README (#7345)

plugins/celery/README.md

@@ -0,0 +1,9 @@
+# Celery
+
+This plugin provides completion for [Celery](http://www.celeryproject.org/).
+
+To use it add celery to the plugins array in your zshrc file.
+
+```bash
+plugins=(... celery)
+```