chore: add an entry to the CHANGELOG

Add an entry about performance counter reset to the CHANGELOG.

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to
 ## [Unreleased]
 
 ### Added
+- [#4987](https://github.com/firecracker-microvm/firecracker/pull/4987): Reset
+  performance counter register on VM startup. This avoids VM reading host
+  performance counter value. This is only possible on 6.4 and newer kernels. For
+  older kernels a warning will be printed to the logs.
 
 ### Changed