Include the README in Autofac NuGet packages

[tillig]

Problem Statement

As of .NET 5 / NuGet 5.10.0 preview 2, you can include your README in the NuGet package to get something richer showing up on the NuGet.org site. Without that, you just get the package description and it looks very sad and lonely in there.

Desired Solution

• Update the README to make sure it works both on the GitHub site and on NuGet.org. I'm not sure I can quantify everything, but, for example...
  • Links to things probably need to be absolute instead of relative to the GitHub repo.
  • Only use the image sources and markdown features they support. Double check that things like badges and the 'Open in VS Code' thing are all on the approved list and are using full URLs.
• Update the .csproj as shown in the instructions to include the README.
• Possibly test this using a manual upload first, to see what it's going to look like. They talk about how to do that on the NuGet.org site. We don't want to iterate over this by actually releasing a bunch of packages.

I don't think this is worth pushing a whole new version for. I think we should just get it ready and then when the next release goes out, this will just be there.

If there are other packages going out sooner, it might be worth doing those first so we get some experience.