Add README and MIT license

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

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+# book-links
+
+A Violentmonkey userscript that shows where else you can get a book while you're browsing one — Anna's Archive and Waterstones — as a small floating widget in the corner of the page.
+
+## Install
+
+1. Install [Violentmonkey](https://violentmonkey.github.io/) (Chrome, Firefox, Edge).
+2. Install the script from the raw file: [book-links.user.js](https://raw.githubusercontent.com/alexture/book-links/main/book-links.user.js) — Violentmonkey will prompt to install it.
+
+The script auto-updates itself from this repo (via `@updateURL`), so you don't need to reinstall it when it changes.
+
+## What it does
+
+On a book page on Bookshop.org, Amazon (`.fr`/`.com`), or Waterstones, it:
+
+1. Extracts the book's ISBN (from the URL or page content) and, as a fallback, title/author.
+2. Checks whether the book is available on:
+   - **Waterstones** — skipped if you're already on Waterstones
+   - **Anna's Archive**
+3. Shows a small widget in the bottom-right corner with a ✓ link if found, a ✗ if confirmed not found, or a `?` link if it couldn't be verified (e.g. JS-rendered search results) — closeable with the × button.
+
+## Supported sites
+
+- `bookshop.org/p/books/*`
+- `waterstones.com/book/*`
+- `amazon.fr` and `amazon.com` product pages (`/dp/*`)
+
+## Notes
+
+- Uses `GM_xmlhttpRequest` to query Anna's Archive and Waterstones directly from the page, bypassing CORS.
+- Availability checks are best-effort text matching against search results, not an official API — the `?` state means "couldn't confirm either way, here's the search link."
+
+## License
+
+MIT — see [LICENSE](LICENSE).