cakephp3: add README (#8196)

plugins/cakephp3/README.md

+# cakephp3 plugin
+
+The plugin adds aliases and autocompletion for [cakephp3](https://book.cakephp.org/3.0/en/index.html).
+
+To use it, add `cakephp3` to the plugins array of your zshrc file:
+```
+plugins=(... cakephp3)
+```
+
+## Aliases
+
+| Alias     | Command                       |
+|-----------|-------------------------------|
+| c3        | `bin/cake`                    |
+| c3cache   | `bin/cake orm_cache clear`    |
+| c3migrate | `bin/cake migrations migrate` |