Add README with book overview and acknowledgments

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: DavidLiedle

README.md

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+# Understanding CORS
+
+**Stop Copy-Pasting Headers and Start Understanding Cross-Origin Resource Sharing.**
+
+Read the book online at [cloudstreet-dev.github.io/Understanding-CORS](https://cloudstreet-dev.github.io/Understanding-CORS/).
+
+## About
+
+This book covers CORS from first principles through production configuration and security. It's structured in five parts:
+
+1. **The Problem CORS Solves** — The Same-Origin Policy, why browsers enforce it, and what an "origin" actually is.
+2. **How CORS Actually Works** — Simple requests, preflights, every header explained, credentials, and caching.
+3. **CORS in the Wild** — Fetch/XHR, WebSockets, fonts, SPAs, API gateways, and proxies.
+4. **Server-Side Configuration** — Express, Go, Rust, Python, Nginx, Apache, serverless, and edge functions.
+5. **Debugging and Security** — Reading error messages, common mistakes, security boundaries, and when `*` is fine.
+
+## Building locally
+
+Install [mdBook](https://rust-lang.github.io/mdBook/), then:
+
+```sh
+mdbook serve --open
+```
+
+## Acknowledgments
+
+Thanks to Georgiy Treyvus, CloudStreet Product Manager, for the idea for this book.
+
+## License
+
+This work is dedicated to the public domain under [CC0 1.0 Universal](LICENSE).