Fix FakeTimeProvider namespace, add readme

[Tratcher]

The Microsoft.Extensions.Time.Testing namespace was mismatched to the package name Microsoft.Extensions.TimeProvider.Testing;.

Added a readme.

Microsoft Reviewers: Open in CodeFlow

[geeknoid]

Note that I intentionally used the Time namespace instead of TimeProvider, since having a type as a namespace component leads to all sorts of weird/unfortunate confusion with the C# compiler.

[Tratcher]

Note that I intentionally used the Time namespace instead of TimeProvider, since having a type as a namespace component leads to all sorts of weird/unfortunate confusion with the C# compiler.

@geeknoid ok, I can appreciate that. We should still re-consider if A) the package and namespace should align, and B) if there would be some other namespace that would be more appropriate, this is the only type in this namespace and it doesn't correlate with the type being tested/extended (System.TimeProvider). I'm not concerned about putting the type in a more common namespace because the type name is unique and the package will only be added to test projects that need it.

Alternatives:

1. Change the namespace to:
   1. System (Matches TimeProvider)
   2. System.Testing (Adjacent to TimeProvider)
   3. System.Time.Testing (Adjacent to TimeProvider, more narrow scope)
2. Rename the package to Microsoft.Extensions.Time.Testing to match the namespace.

[Tratcher]

I've reverted the namespace change for now.

[joperezr]

I've reverted the namespace change for now.

Do you plan on tracking that in the same fit & finish workitem or want to log a new one in favor of closing the fit & fnish one? I'm fine either way, as long as we make sure we revisit this in the next couple of weeks 😃

[Tratcher]

Using the same work item is easier.