cake: add README (#7473)

plugins/cake/README.md

+# Cake
+
+This plugin provides completion for [CakePHP](https://cakephp.org/).
+
+To use it add cake to the plugins array in your zshrc file.
+
+```bash
+plugins=(... cake)
+```
+
+## Note
+
+This plugin generates a cache file of the cake tasks found, named `.cake_task_cache`, in the current working directory. 
+It is regenerated when the Cakefile is newer than the cache file. It is advised that you add the cake file to your
+`.gitignore` files.