Cross-Unity Dependencies Issues/Documentation Request

[andreiborodin]

The following two items were brought forward from #74 into their own issue regarding Cross-Unity dependencies scenarios:

1. Documentation should call out that all code created for consumption should be placed in an asmdef. Trying to reference an Assembly-CSharp project from another Assembly-CSharp project generates issues.

2. It seemed that project references needed to be duplicated in both the asmdef you care about's project as well as the Dependencies project when making references to other unity projects.