Add section on global usings

[mvonballmo]

Notes

Library authors should be aware that package users will generally only see the source files downloaded via SourceLink. If too many of the usings are in a solution-specific "global usings" file, then it won't be clear from the sources where the types came from. The compiler will know, but the developer will not. This applies also to direct developers of the project.

It almost goes without saying that you should not include global usings that might cause ambiguities in too many source files.