chore: move and document rate limit middleware

middleware/rate-limit/README.md

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+# Middleware: Rate Limiting
+
+This example showcases two middleware that achieve rate limiting for your API endpoints. One uses a simple leaky-bucket implementation and the other delegates to [`actix-governor`].
+
+## Usage
+
+```sh
+cd middleware/rate-limit
+cargo run
+```
+
+Look in `src/rate_limit.rs` to see the leaky-bucket implementation.
+
+## Routes
+
+- [GET /test/simple](http://localhost:8080/test/simple) - uses the hand-written leaky-bucket rate limiting.
+- [GET /test/governor](http://localhost:8080/test/governor) - uses [`actix-governor`].
+
+Calling either of these endpoints too frequently will result in a 429 Too Many Requests response.
+
+[`actix-governor`]: https://crates.io/crates/actix-governor