rand_chacha: update changelog

Author: kazcw

rand_chacha/CHANGELOG.md

@@ -5,6 +5,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+- improve AVX2 performance by about 16% (CC20/Broadwell) to 48% (CC8/Zen3) (#1192)
 - Made `rand_chacha` propagate the `std` feature down to `rand_core`
 
 ## [0.3.1] - 2021-06-09