Add changelog entry for signed-digit ecmult_const algorithm

Author: sipa

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 #### Changed
  - When consuming libsecp256k1 as a static library on Windows, the user must now define the `SECP256K1_STATIC` macro before including `secp256k1.h`.
+ - The point multiplication algorithm used for ECDH operations (module `ecdh`) was replaced with a slightly faster one.
 
 ## [0.3.2] - 2023-05-13
 We strongly recommend updating to 0.3.2 if you use or plan to use GCC >=13 to compile libsecp256k1. When in doubt, check the GCC version using `gcc -v`.