Document option for SXG-only subresource preloads (#177)
Document option for SXG-only subresource preloads (#177)

I'm not exactly sure what the use case for this is, yet, but with [Priority Hints](https://web.dev/priority-hints/) landing, I expect there may eventually be diverging use cases for preloads when serving SXG vs non-SXG.
google/sxg-rs,google/sxg-rs,google/sxg-rs,google/sxg-rs,google/sxg-rs,google/sxg-rs