Provide guidance in VS 2015 about applying "sdk": "version" in global.json

[TheRealPiotrP]

Moving from https://github.com/dotnet/cli/issues/5375 on behalf of @DustinCampbell

---

We should add guidance to help users when a newer .NET CLI is on the path that does not support project.json. I expect that project.json and I will be with us for a long while as not everyone will upgrade overnight.

Clearly, this would require servicing VS 2015 tooling which we likely wouldn't do just for this issue. However, if we do choose to service VS 2015 tooling, we should consider doing this at the same time.