Add readme for React Aria Components npm package (#4331)

Author: devongovett

packages/react-aria-components/README.md

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+# [React Aria Components](https://react-spectrum.adobe.com/react-aria/react-aria-components.html)
+
+React Aria Components is a library of unstyled components implementing [ARIA patterns](https://www.w3.org/WAI/ARIA/apg/), built on top of the [React Aria](https://react-spectrum.adobe.com/react-aria/index.html) hooks. It provides components for common UI patterns, with accessibility, internationalization, interactions, and behavior built in, allowing you to focus on your unique design and styling rather than re-building these challenging aspects. React Aria has been meticulously tested across a wide variety of devices, interaction modalities, and assistive technologies to ensure the best experience possible for all users.
+
+Compared with the React Aria hooks, React Aria Components provides a default DOM structure and styling API, and abstracts away the glue code necessary to connect the hooks together. The components and hooks also work together, allowing them to be mixed and matched depending on the level of customization required.
+
+## Features
+
+* ♿️ **Accessible** – React Aria Components provides accessibility and behavior according to [WAI-ARIA Authoring Practices](https://www.w3.org/TR/wai-aria-practices-1.2/), including full screen reader and keyboard navigation support. All components have been tested across a wide variety of screen readers and devices to ensure the best experience possible for all users.
+* 📱 **Adaptive** – React Aria Components ensures consistent behavior, no matter the UI. It supports mouse, touch, keyboard, and screen reader interactions that have been tested across a wide variety of browsers, devices, and platforms.
+* 🌍 **International** – React Aria Components supports over 30 languages, including right-to-left-specific behavior, internationalized date and number formatting, and more.
+* 🎨 **Styleable** – React Aria Components does not include any styles by default, allowing you to build custom designs to fit your application or design system, with any styling methodology.
+
+## Status
+
+React Aria Components is currently in **alpha**. This means APIs will likely change in future updates as we discover the best ways to use it, and there are some known bugs and limitations. That said, it is based on a solid and battle-tested foundation in React Aria, and we would love for you to try it out and give us feedback! This will directly help us shape the APIs and make it the best library it can be. Please report issues and feature requests [on GitHub](https://github.com/adobe/react-spectrum/issues).
+
+## Documentation
+
+You can learn more about React Aria Components in our [documentation](https://react-spectrum.adobe.com/react-aria/react-aria-components.html).