Update changelog

Author: diondokter

cortex-m-rt/CHANGELOG.md

@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Add `zero-init-ram` feature to initialize RAM with zeros on startup. This can be necessary on
   safety-critical hardware to properly initialize memory integrity measures.
+- Add `hardfault-trampoline` feature that is enabled by default which enables
+  the previously required hardfault trampiline. Now when this feature is not active, there will be
+  no trampoline and the function you write is the exact function that is used as hardfault handler.
 
 ## [v0.7.3]