update changelog for 2.8

Author: jhunsaker

CHANGELOG.md

@@ -5,6 +5,9 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [2.8] - 2021-10-05
+- [pyth] use some bits for storing exponent to avoid overflow (twap)
+
 ## [2.7] - 2021-10-02
 - [pyth] proper lprice/uprice sorting in confidence calculation