Add changelog entry for signed-digit ecmult_const algorithm

Author: sipa

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 #### Changed
  - Forbade cloning or destroying `secp256k1_context_static`. Create a new context instead of cloning the static context. (If this change breaks your code, your code is probably wrong.)
  - Forbade randomizing (copies of) `secp256k1_context_static`. Randomizing a copy of `secp256k1_context_static` did not have any effect and did not provide defense-in-depth protection against side-channel attacks. Create a new context if you want to benefit from randomization.
+ - The point multiplication algorithm used for ECDH operations (module `ecdh`) was replaced with a slightly faster one.
 
 ## [0.2.0] - 2022-12-12