Update guidance on using VS NuGet UX

[andreiborodin]

The NuGet UX can be used to manage the NuGet references for generated projects, however, we need to confirm the following two items for us:

• Entries can get added below the import, is that fine or not?
• Should we recommend using the UX?

[andreiborodin]

We are recommending using the UX, but also asking the user to verify that entries are added above the import. This has been added as part of #122.

In the future, we should try to remove the bottom import, as the NuGet UX does put it below periodically, and we do have stuff in there that currently requires to be run after all package/project references.