README.md: add link to RSA repo

README.md

@@ -18,6 +18,8 @@ and can be easily used for bare-metal or lightweight WebAssembly programming.
 | [`ed25519`] | [Ed25519](https://en.wikipedia.org/wiki/EdDSA) | [![crates.io](https://img.shields.io/crates/v/ed25519.svg)](https://crates.io/crates/ed25519) | [![Documentation](https://docs.rs/ed25519/badge.svg)](https://docs.rs/ed25519) | [![ed25519 build](https://github.com/RustCrypto/signatures/workflows/ed25519/badge.svg?branch=master&event=push)](https://github.com/RustCrypto/signatures/actions?query=workflow%3Aed25519)
 | [`rfc6979`] | [RFC6979](https://datatracker.ietf.org/doc/html/rfc6979) | [![crates.io](https://img.shields.io/crates/v/rfc6979.svg)](https://crates.io/crates/rfc6979) | [![Documentation](https://docs.rs/rfc6979/badge.svg)](https://docs.rs/rfc6979) | [![rfc6979 build](https://github.com/RustCrypto/signatures/actions/workflows/rfc6979.yml/badge.svg)](https://github.com/RustCrypto/signatures/actions/workflows/rfc6979.yml)
 
+NOTE: for RSA signatures see <https://github.com/RustCrypto/RSA>
+
 ## Usage
 
 Crates functionality is expressed in terms of traits defined in the [`signature`][2]