Update changelog for DWT::get_cycle_count deprecation

Author: newAM

CHANGELOG.md

@@ -14,6 +14,13 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
     - LSU counter
     - Folded-instruction counter
 
+### Deprecated
+
+- `DWT::get_cycle_count` has been deprecated in favor of `DWT::cycle_count`.
+  This change was made for consistency with the [C-GETTER] convention. (#349)
+
+[C-GETTER]: https://rust-lang.github.io/api-guidelines/naming.html#c-getter
+
 ## [v0.7.3] - 2021-07-03
 
 ### Fixed