Add note to CHANGELOG

Author: jgallagher

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 
 * Fixed `pool` example in docstring.
+* Fixed undefined behavior in `Vec::truncate()`, `Vec::swap_remove_unchecked()`,
+  and `Hole::move_to()` (internal to the binary heap implementation).
 
 ### Added