improve extensibility docs

[zivkan]

You have some docs here: https://github.com/microsoft/VSProjectSystem/blob/master/doc/extensibility/

1. This directory (and a few others) have Index.md files, but if they're renamed to Readme.md, then GitHub will render the contents when browsing the folder, which will make them much more discoverable.

2. When my vsix project references the latest Microsoft.VisualStudio.SDK package, these APIs are not available. I haven't seen in the docs where the package name is specified.

   • I thought it might be Microsoft.VisualStudio.ProjectSystem.Sdk, because it's in the sample project, but my IntelliSense won't find IProjectContextMenuProvider.
   • On nuget.org, there's a package Microsoft.VisualStudio.ProjectSystem.Managed, but it's version 2.0, uploaded in 2017. Looking at the NuGet.Client repo I found a feed where newer versions are published. But the feed url for up to date versions of the package should be documented if it's not on nuget.org.

3. I was trying to implement IProjectContextMenuProvider. The docs say that you need a working command first, and I've been able to get a command working int he Tools menu, on the Solution Explorer toolbar, and even on the solution node and a project node, all using the vsct file. I tried implementing the interface, but when I return true and set the command guid and id, the context menu just doesn't appear. So, my code seems to trigger some kind of error in VS, but there's no error displayed anywhere, and I have no idea what else to try. A working sample would be great, otherwise I think the docs for this interface should be improved so customers can be successful in implementing it.