changelog: add entry for "field: Remove x86_64 asm"

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 #### Changed
  - The point multiplication algorithm used for ECDH operations (module `ecdh`) was replaced with a slightly faster one.
+ - Optional x86-64 assembly for field operations was removed because modern C compilers are able to output more efficient assembly. This change results in a significant speedup of some library functions when compiled with the `--with-asm` option (enabled by default) on x86-64. Benchmarks with GCC 10.5.0 show a 10% speedup for `secp256k1_ecdsa_verify` and `secp256k1_schnorrsig_verify` (see https://github.com/bitcoin-core/secp256k1/issues/726#issuecomment-1824625653).
+
 
 ## [0.4.0] - 2023-09-04