README: update first sentence

libsecp256k1 has become more than a library for just ECDSA and key tweaking.

README.md

@@ -5,7 +5,7 @@ libsecp256k1
 ![Dependencies: None](https://img.shields.io/badge/dependencies-none-success)
 [![irc.libera.chat #secp256k1](https://img.shields.io/badge/irc.libera.chat-%23secp256k1-success)](https://web.libera.chat/#secp256k1)
 
-Optimized C library for ECDSA signatures and secret/public key operations on curve secp256k1.
+High-performance high-assurance C library for digital signatures and other cryptographic primitives on the secp256k1 elliptic curve.
 
 This library is intended to be the highest quality publicly available library for cryptography on the secp256k1 curve. However, the primary focus of its development has been for usage in the Bitcoin system and usage unlike Bitcoin's may be less well tested, verified, or suffer from a less well thought out interface. Correct usage requires some care and consideration that the library is fit for your application's purpose.