Document dev setup

[reedy]

It wasn't the most straight forward when I had to do it again many years later.

Reminder would be useful for future

[reedy]

Install IntelliJ IDEA Community/Ultimate

Make sure Plugin DevKit plugin is enabled

Get from version control

File -> Project Structure. Under Project SDK make sure you install/select an SDK. Then “Add IntelliJ Platform Plugin SDK”, browse to PhpStorm

Add another classpath. Plugins, CSS/JavaScriptLanguage/Php

[reedy]

^ is completely out of date now...