Add Contributing Section to Readme

[CollinHerber]

As a very long time Jetbrain and Aurelia enjoyer I have longed for a plugin that made Aurelia development within IntelliJ as good as some of the more known, but obviously worse, frameworks.

While AureliaStorm helps, I feel it's missing a lot of things and I want to start contributing to the project.

Having contribution docs will help not only me, but anyone else, looking to contribute.
I am new to plugin development myself so having a breakdown of building, testing, etc would be nice.

Additionally, I am using Aurelia 2 so once I figure all this out I hope to add support around Aurelia 2.