Add embedded READMEs to NuGet packages

[ajcvickers]

See:

• Introducing the New NuGet.org Package Details Page
• Sample Microsoft.EntityFrameworkCore readme
• Documentation for PackageReadme

[ajcvickers]

/cc @Pilchie @loic-sharma @leecow Would be good to be able to do this in the 6.0.1 patch release.

[loic-sharma]

Let us know if you need any help at all! /cc @chgill-MSFT

[ajcvickers]

Assigning to @bricelam, @JeremyLikness, and myself. If one of you starts work on this, then please let the others know. Would be good to get in by the 15th for 6.0.1, but not critical.