docs: add README to playground

Author: bigmistqke

playground/README.md

@@ -0,0 +1,29 @@
+# solid-three Tutorials Playground
+
+A local Vite app for exploring `solid-three` API demos and examples interactively.
+
+## Running
+
+```bash
+pnpm install
+pnpm dev
+```
+
+## Structure
+
+```
+src/
+  api/          # Per-feature API demos, one folder per concept
+  examples/     # Larger composed examples (solar system, environment, …)
+controls/       # Shared orbit-controls and other helpers
+```
+
+The sidebar groups **API** demos by category and lists **Examples** separately. Each API category maps to a folder under `src/api/`; each folder typically contains a `usage.tsx` entry point plus additional focused demo files.
+
+## Adding a Demo
+
+**API demo** — create a `.tsx` file under `src/api/<category>/`. It is picked up automatically via Vite glob import and appears in the sidebar.
+
+**Example** — create a `.tsx` file under `src/examples/`. Same auto-discovery applies.
+
+Each file should export a default component that renders the demo (with its own `<Canvas>` if needed).