Adding Reference to Myra

There are two ways of referencing Myra in the project:

1. Through nuget: install-package Myra for MonoGame(or install-package Myra.FNA for FNA)

2. Through source code:

   a. Add Myra source code to the project(i.e. as git submodule).

   b. Copy SolutionDefines.targets from build/MonoGame(or build/FNA) to the solution folder.

   c. Add src/Myra/Myra.csproj to the solution.