[Proposal] Make Directory.Build.props more discoverable

See https://github.com/microsoft/msbuild/issues/762 for a lot of good discussion on the pain people have with this file.

I love this feature, but the "magic" involved in it turns people off. We have people on our team who want to explicitly ban the use of this feature because of it.

# Proposal
Have `dotnet new` and VS Create New Project scenarios place an empty Directory.Build.props file next to the project with a high level comment about what it does and then a link to more documentation. This way developers would at least be familiar with it even when they've never used it.