docs: add README to examples root (#4)

When linking to mdx examples via
https://github.com/marimo-team/mdx-marimo/tree/main/examples we want
visitors to be greeted with a README instead only the gh file tree.

Author: peter-gy

examples/README.md

@@ -0,0 +1,18 @@
+# Examples
+
+These examples add reactive marimo cells to MDX pages in several hosts.
+Each folder contains a runnable integration.
+
+| Host           | Example                                |
+| -------------- | -------------------------------------- |
+| Astro          | [`with-astro`](./with-astro)           |
+| Docusaurus     | [`with-docusaurus`](./with-docusaurus) |
+| Next.js        | [`with-next`](./with-next)             |
+| Nuxt           | [`with-nuxt`](./with-nuxt)             |
+| React and Vite | [`with-react`](./with-react)           |
+| Vue and Vite   | [`with-vue`](./with-vue)               |
+
+Use the same integration pattern with any host that supports
+[MDX](https://mdxjs.com/): add `remarkMarimo` to the host's MDX compiler, load
+the package stylesheet, and register the browser runtime. See
+[framework setup](../docs/api/frameworks.mdx) for the shared contract.