[Doc] Add doc about integrating with renderToPipeableStream

[alicerocheman]

Usage of Suspense forces me to use renderToPipeableStream, but I cannot find any doc anywhere on how renderToPipeableStream can integrate with react-helmet-async.

I tried using stuff from the react-helmet-async README where it explains usage with renderToNodeStream (which is deprecated) but it's missing some info (ie what is the "template" function and how is it declared?)

As is, react-helmet-async documentation does not tell me how to integrate with current react's Suspense.