eecms: add README (#8277)

plugins/eecms/README.md

+# eecms plugin
+
+This plugin adds auto-completion of console commands for [`eecms`](https://github.com/ExpressionEngine/ExpressionEngine).
+
+To use it, add `eecms` to the plugins array of your `.zshrc` file:
+```
+plugins=(... eecms)
+```
+
+It also adds the alias `eecms` which finds the eecms file in the current project
+and runs it with php.