Document the ProjectReference protocol (#1892)
Document the ProjectReference protocol (#1892)

* Document the ProjectReference protocol

Wrote a document explaining what ProjectReferences do behind the scenes
and how to make a green-field project that doesn't import the machinery
of Common.targets compatible with using them.
mono/msbuild,AndyGerlicher/msbuild,mono/msbuild,mono/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,Microsoft/msbuild,jeffkl/msbuild,chipitsine/msbuild,jeffkl/msbuild,rainersigwald/msbuild,cdmihai/msbuild,Microsoft/msbuild,jeffkl/msbuild,chipitsine/msbuild,chipitsine/msbuild,rainersigwald/msbuild,jeffkl/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,mono/msbuild,rainersigwald/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,cdmihai/msbuild