CHANGELOG: const 'PTR' is introduced for Core Peripherals

Author: JOE1994

CHANGELOG.md

@@ -11,6 +11,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - New `InterruptNumber` trait is now required on interrupt arguments to the
   various NVIC functions, replacing the previous use of `Nr` from bare-metal.
+- Associated const `PTR` is introduced to Core Peripherals to
+  replace the existing `ptr()` API. `ptr()` API is to be removed in the
+  next breaking release (v0.7)
 
 ## [v0.6.2] - 2020-01-12