fix(docs): add note in readme about loose typing of useSnapshot

[dai-shi]

close #327

---

With #359, we can do Module Augmentation.

https://codesandbox.io/s/react-typescript-forked-qphpi?file=/src/App.tsx:46-125

declare module "valtio" {
  function useSnapshot<T extends object>(p: T): T;
}

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[dai-shi]

Hmm, I wonder if Module Augmentation works in this case.

[github-actions]

Size Change: 0 B

Total Size: 34.5 kB

ℹ️ View Unchanged

Filename	Size
dist/esm/index.js	1.36 kB
dist/esm/macro.js	613 B
dist/esm/utils.js	3.91 kB
dist/esm/vanilla.js	1.84 kB
dist/index.js	1.45 kB
dist/macro.js	910 B
dist/system/index.js	1.59 kB
dist/system/macro.js	721 B
dist/system/utils.js	4.14 kB
dist/system/vanilla.js	1.98 kB
dist/umd/index.js	1.59 kB
dist/umd/macro.js	1.06 kB
dist/umd/utils.js	4.71 kB
dist/umd/vanilla.js	2.1 kB
dist/utils.js	4.54 kB
dist/vanilla.js	1.96 kB

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