Docs: NUnit integration

[pl-aknight]

Under a doc section named "Usage", write a doc page to show how to use Boa Constrictor with NUnit.

The page should include:

1. Creating an NUnit project
2. Adding the Boa Constrictor NuGet package
3. Writing a basic test with Boa Constrictor

Reference the tutorial, since it uses NUnit. The purpose of this page is to be a quick reference, not a lengthy tutorial itself. It should somewhat mirror the SpecFlow page for #88.