Add readme to source

Co-authored-by: Alex Sanders <alex@sndrs.dev>

libs/@guardian/source/README.md

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+# `@guardian/source`
+
+<img src="assets/logo.png" height="320" width="620" >
+
+> An NPM package containing design foundations and robust, accessible React components from the Guardian's
+> [Source Design System](https://theguardian.design).
+
+[![npm](https://img.shields.io/npm/v/@guardian/source)](https://www.npmjs.com/package/@guardian/source)
+<a href="https://guardian.github.io/storybooks/?path=/docs/source" target="_blank"><img src="https://raw.githubusercontent.com/storybooks/brand/master/badge/badge-storybook.svg"></a>
+
+## Install
+
+```sh
+$ pnpm add @guardian/source
+```
+
+or
+
+```sh
+$ yarn add @guardian/source
+```
+
+or
+
+```sh
+$ npm install @guardian/source
+```
+
+> [!NOTE]
+> Use of the React components will require [@emotion/react](https://emotion.sh/docs/introduction#react)
+
+## Documentation
+
+Full documentation for components and foundations
+is available in the [Source storybook](https://guardian.github.io/storybooks/?path=/docs/source).
+
+## Contributing
+
+We welcome contributions to Source! See our [contributing](../../../docs/source/contributing.md) and [Storybook](../../../docs/source/storybook.md) docs for more info.