Update CHANGELOG.md

Author: jonathanpallant

cortex-m/CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 - MSRV is 1.61 to match cortex-m-rt crate
+- Add `enter_unprivileged` function to switch to unprivileged mode (on the Process Stack, or `PSP`)
 
 ## [v0.7.7] - 2023-01-03