Add usage tracking capability

[dai-shi]

use-valtio/README.md

Lines 56 to 57 in 9d01480

	There's a drawback (but it can be a benefit) with `useValtio`.
	Unlike `useSnapshot`, `useValtio` doesn't have usage tracking capability.

I thought it might be interesting with explicit dependency (and #3 supports it), Valtio comes with usage tracking, so use-valtio should do the same for comparison.

Originally, as there was a compatibility issue with memo, explicit dependency might have seemed good, but now it can be solved pmndrs/valtio#866, let's do the same.