compleat: add README (#8236)

plugins/compleat/README.md

@@ -0,0 +1,8 @@
+# compleat plugin
+
+This plugin looks for [compleat](https://github.com/mbrubeck/compleat) and loads its completion.
+
+To use it, add compleat to the plugins array in your zshrc file:
+```
+plugins=(... compleat)
+```