Add a demo for ProduceReferenceAssembly

[KirillOsenkov]

This is an awesome effort!

Would also be nice to add the next step demo: have two projects, one referencing the other, and have them set ProduceReferenceAssembly.

This way the referencing project will actually get the .dll from ref folder, which strips method bodies and is a reference assembly. This way you can make a whitespace change, or a method-body change or private API change in the first project, and the second project will not rebuild.

[KirillOsenkov]

Now I'm thinking this belongs elsewhere since this repo is focused on source link and reproducible builds specifically.