docs: add an example for setting the state outside event (#302) (#303)

* feat(docs): add an example for setting the state outside event (#302)

* fix gramar issues

* Point more details URL to the issue instead of the tweet

* Simplify example

Co-authored-by: Daishi Kato <dai-shi@users.noreply.github.com>

* Update readme.md

* Update readme.md

Co-authored-by: Cristian Ratoi <cristian.ratoi@everymatrix.com>
Co-authored-by: Daishi Kato <dai-shi@users.noreply.github.com>

readme.md

@@ -363,6 +363,28 @@ import { redux } from 'zustand/middleware'
 const useStore = create(redux(reducer, initialState))
 ```
 
+## Calling actions outside a React event handler
+
+Because React handles `setState` synchronously if it's called outside an event handler. Updating the state outside an event handler will force react to update the components synchronously, therefore adding the risk of encountering the zombie-child effect.
+In order to fix this, the action needs to be wrapped in `unstable_batchedUpdates`
+
+```jsx
+import { unstable_batchedUpdates } from 'react-dom' // or 'react-native'
+
+const useStore = create((set) => ({
+  fishes: 0,
+  increaseFishes: () => set((prev) => ({ fishes: prev.fishes + 1 }))
+}))
+
+const nonReactCallback = () => {
+  unstable_batchedUpdates(() => {
+    useStore.getState().increaseFishes()
+  })
+}
+```
+
+More details: https://github.com/pmndrs/zustand/issues/302
+
 ## Redux devtools
 
 ```jsx