Document how to migrate items and properties from legacy Xamarin projects to .NET projects

[rolfbjarne]

See https://github.com/xamarin/xamarin-macios/wiki/Project-file-properties-dotnet-migration.

Upgrade assistant: https://github.com/dotnet/upgrade-assistant

[rolfbjarne]

@chamons in d7c2a45 you added the BundleDependentFiles item group as an input to the _CompileToNative tasks, but it doesn't seem like we use that item group anywhere else (nor adding to it) - was there any specific reason for this addition? Can it be removed? Or described/documented?

[chamons]

@rolfbjarne - I have honestly no clue. I think they can be removed based upon reviewing the PR.

[rolfbjarne]

This is mostly documented now, at least indirectly, in https://github.com/xamarin/xamarin-macios/wiki/Project-file-properties.