Add tracesSampleRate to the main initialize RN SDK code snippet (#6640)

Author: krystofwoldrich

src/platform-includes/getting-started-config/react-native.mdx

@@ -7,6 +7,11 @@ import * as Sentry from "@sentry/react-native";
 
 Sentry.init({
   dsn: "___PUBLIC_DSN___",
+
+  // Set tracesSampleRate to 1.0 to capture 100%
+  // of transactions for performance monitoring.
+  // We recommend adjusting this value in production
+  tracesSampleRate: 1.0,
 });
 ```