reason-react-native: example for AppState docs (facebook#589)

* Docs for Appstate

I lost whatever pull request that was so I have recreated here in response to issue facebook#564

* Update reason-react-native/src/apis/AppState.md

Co-Authored-By: Max Thirouin <[email protected]>

reason-react-native/src/apis/AppState.md

@@ -48,3 +48,46 @@ external removeEventListener:
   "";
 
 ```
+
+# Example
+
+The example below illustrates rendering to the ui based on the value returned from the `AppState` API.
+
+```reason
+[@react.component]
+  let make = () => {
+    let (appState, setAppState) = React.useState(_ => AppState.currentState);
+
+    let handleAppStateChange = nextAppState => {
+      switch (appState, nextAppState) {
+      | (_, state) when state === AppState.background =>
+        Js.log("App has come to the background!")
+      | (_, state) when state === AppState.active =>
+        Js.log("App has come to the foreground!")
+      | _ => ()
+      };
+      setAppState(_ => nextAppState);
+    };
+
+    React.useEffect(() => {
+      AppState.addEventListener(
+        `change(state => handleAppStateChange(state)),
+      )
+      Some(
+        () =>
+          AppState.removeEventListener(
+            `change(state => handleAppStateChange(state)),
+          ),
+      );
+    });
+
+    let renderAppState =
+      switch (appState) {
+      | appState when appState === AppState.active => "active"
+      | appState when appState === AppState.background => "background"
+      | appState when appState === AppState.inactive => "inactive"
+      | _ => "unknown"
+      };
+    <Text> {"Current state is: " ++ renderAppState |> React.string} </Text>;
+  };
+```