Add file-scoped namespace note to README.md

[qJake]

There are some folks that use external tools (such as ReSharper) instead of the built-in VS code cleanup tools.

To that end, I didn't know that this extension supported file-scoped namespaces. I found the information after digging through some open issues/comments.

I added a note to the README.md in case folks want to know how you can enable support for file-scoped namespaces (without the need to create custom templates).

[dahlsailrunner]

Thanks for this PR! Helped me discover the feature! I was wondering if there was any way to do this. :)

Hopefully it gets merged.