add app state refresh cookbook

Author: roncohen

packages/react-native-sdk/README.md

@@ -27,3 +27,43 @@ import { ReflagProvider, useFlag } from "@reflag/react-native-sdk";
   <MyApp />
 </ReflagProvider>;
 ```
+
+## Cookbook
+
+### Refresh flags when the app returns to the foreground
+
+```tsx
+import React, { useEffect, useRef } from "react";
+import { AppState } from "react-native";
+import { ReflagProvider, useClient } from "@reflag/react-native-sdk";
+
+function AppStateListener() {
+  const client = useClient();
+  const appState = useRef(AppState.currentState);
+
+  useEffect(() => {
+    const subscription = AppState.addEventListener("change", (nextAppState) => {
+      if (
+        appState.current.match(/inactive|background/) &&
+        nextAppState === "active"
+      ) {
+        void client.refresh();
+      }
+      appState.current = nextAppState;
+    });
+
+    return () => subscription.remove();
+  }, [client]);
+
+  return null;
+}
+
+export function App() {
+  return (
+    <ReflagProvider publishableKey="{YOUR_PUBLISHABLE_KEY}">
+      <AppStateListener />
+      <MyApp />
+    </ReflagProvider>
+  );
+}
+```