[docs] Visual Studio publishing and <RuntimeIdentifiers>

[MV10]

This is a documentation/readme suggestion based on AvaloniaUI/Avalonia#1665 (comment)

The SkiaSharp NuGet package offers runtime versions such as win7-x64 and win10-x64, but by default Visual Studio 2017's Publish options only list fallback runtime identifiers such as win-x64. The result is a missing libSkiaSharp.dll and the Unable to load DLL exception.

The solution is simple but not necessarily obvious: add a <RuntimeIdentifiers> node to the csproj, which expands the available RIDs in the VS Publish dialog.

I'm suggesting this and the specific available runtime versions should be documented, especially given that the readme mentions the same exception but attributes it to a different problem, which could be misleading and confusing.

[mattleibow]

Thanks for pointing this out. I updated the RID and should be in the next major release.