Create maps-and-sets-usage.md (#997)

* Create maps-and-sets-usage.md

Hi, there. I was using this lib with React and I found that there is a specific way to use Maps and Sets, so I thought that a little update could help others. What do you guys think?

* Update docs/maps-and-sets-usage.md

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

* Update maps-and-sets-usage.md

The size went down cause using that way there's no going wrong...
Maybe it is still a good thing to have so others don't lose time like me??

* Update maps-and-sets-usage.md

* Update maps-and-sets-usage.md

Is it like this?

* run prettier

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

docs/maps-and-sets-usage.md

@@ -0,0 +1,24 @@
+## Map and Set Usage
+
+You need to wrap Maps and Sets inside an object, and when you want it's update to be reflected (e.g. in React),
+you do it by calling the setState on it:
+
+**You can view a codesandbox here: https://codesandbox.io/s/late-https-bxz9qy**
+
+```js
+import create from 'zustand'
+
+const useFooBar = create(() => ({ foo: new Map(), bar: new Set() }))
+
+function doSomething() {
+  // doing something...
+
+  // If you want to update some React component that uses `useFooBar`, you have to call setState
+  // to let React know that an update happened.
+  // Following React's best practices, you should create a new Map/Set when updating them:
+  useFooBar.setState((prev) => ({
+    foo: new Map(prev.foo).set('newKey', 'newValue'),
+    bar: new Set(prev.bar).add('newKey'),
+  }))
+}
+```