Determine desired TFM behavior in a product build (+document?)

[dagood]

In #181 (comment) @eerhardt had some good questions about the source-built product:

[An arg [to tell a repo what TFM to build] makes sense to me.] It sounds like we'll need one for netstandard and one for netcoreapp.

Do we need both? I assumed the idea was that the whole source build would be for netcoreapp? /cc @weshaggard @ericstj

A really good/important question we need to figure out is: "What is the expected usage of these source-built assets?"

Do we expected people to reference the .nupkgs that are produced by this build? If so, then they won't be able to build any netstandard1.x projects. (Or even netstandard2.0 when we move to a netstandard2.1 or 3.0.) Is this intended?

I'm not 100% sure this is something that needs to be independently tracked/documented, but it's easy to close this issue if not.

[dseefeld]

This will come into play when we address #210.

[weshaggard]

Lets use #210 to track