Add a basic "getting started" guide

Hello again!

I'm a newcomer to Unity, and I found it a little bit confusing to get The Intercept up-and-running in the Unity editor. At first I was worried that I might have imported the wrong folder, or that I was using the wrong version of Unity, or that something else had gone amiss. I did eventually figure things out, though!

Anyhow, I'm not sure how many people who are interested in Ink but unfamiliar with Unity will be looking at this project, but if you think it'd be helpful, I could try writing a very basic "getting started" guide--either as a markdown file in this repository or perhaps a wiki page.