chore: update docs

README.md

@@ -11,6 +11,18 @@ Implementation of [IETF RFC 9421](https://datatracker.ietf.org/doc/html/rfc9421)
 
 This crates provides a basic library [httpsig](./httpsig) and [its extension](./httpsig-hyper/) of `hyper`'s http library. At this point, our library can sign and verify only request messages of hyper. (TODO: response message signature)
 
+## Supported Signature Algorithms
+
+- [x] HMAC using SHA-256
+- [x] Ed25519
+- [x] ECDSA-P256 using SHA-256
+- [ ] ECDSA-P384 using SHA-384
+
+~~- [ ] RSASSA-PSS using SHA-512~~
+~~- [ ] RSASSA-PKCS1-v1_5 using SHA-256~~
+
+At this point, we have no plan to support RSA signature due to [the problem related to the non-constant time operation](https://github.com/RustCrypto/RSA/issues/19), i.e., Mervin Attack.
+
 ## Usage of Extension for `hyper` (`httpsig-hyper`)
 
 This is a case signing and verifying a signature generated with asymmetric cryptography (like EdDSA), where `PUBLIC_KEY_STRING` and `SECRET_KEY_STRING` is a public and private keys in PEM format, respectively. Generating and verifying a MAC through symmetric crypto (HMAC-SHA256) is also supported.