Add "getting started" section to README.

[toolness]

View the formatted guide here.

This fixes #2 by adding a "getting started" guide to the README. I decided to write this because someone contacted me via email expressing an interest in having such a guide--so at least it will be useful to someone out there! :grin:

Learning objectives of the guide:

• The learner should become acquainted with the Game, Heirarchy, Inspector, and Project windows of Unity.
• The learner should become acquainted with the Ink Player window of ink-unity-integration.
• The learner should understand how to tinker with the .ink script and see their changes (a) in-game and (b) in the Ink Player.

Learning objectives not addressed (but can be added via future PRs):

• The learner should become acquainted with the Console window of Unity--specifically, so they know where to look when their .ink script contains errors. (Note that this can be taught much more easily if https://github.com/inkle/ink-unity-integration/issues/15 is fixed.)
• The learner should know how to build their project and share it with others.
• The learner should understand how to change C# code.