Mention how to extend (most) any type

I had an "aha moment" when I read https://github.com/Microsoft/TypeScript/issues/16985#issuecomment-318833845

I think it would be great to explain somewhere that if you want to extend a literal interface, just give it a name. I don't see a good place for this tip; otherwise, I'd draft a PR.

Start by reading the linked TypeScript issue comment and locating the book section that discusses extending literal interfaces. Add a concise explanation that naming a literal interface makes it extendable, with the wording placed where readers will find this guidance.】【。

Written by the indexing model from the issue text.