docs(readme): lead with what it is, where it fits and how to upgrade

Moves the development notes and the how-it-works paragraph to
CONTRIBUTING.md so the README covers install, quickstart, reference,
Interchange wiring and the 0.1 upgrade path. Every tsx block
typechecks against the packed build.

Author: TheGreatAxios

CONTRIBUTING.md

@@ -5,6 +5,14 @@ registry and developed with [bun](https://bun.sh) as its package manager and tas
 Read [ARCHITECTURE.md](./ARCHITECTURE.md) first — most of the rules below only make sense
 once that is clear.
 
+## How it works
+
+Components never fetch; data arrives as props. The root entry is generated re-exports
+only, and the package is side-effect free apart from its CSS, so either import style
+tree-shakes. Tokens and keyframes live in `theme.css`; `styles.css` is the prebuilt sheet
+for hosts that do not run Tailwind. [PRODUCT.md](./PRODUCT.md) covers why the library
+exists and [IMPLEMENTATION.md](./IMPLEMENTATION.md) the toolchain and peers.
+
 ## Getting set up
 
 ```sh