Add mising item to the change log

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 
+- `Mtvec::trap_mode()` not returns `Option<TrapMode>` (breaking change)
 - Updated Minimum Supported Rust Version to 1.31.0
 - Use `llvm_asm!` instead of `asm!`