Update changelog

Author: teskje

CHANGELOG.md

@@ -13,6 +13,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - SPI support for reclock after initialization ([#98](https://github.com/stm32-rs/stm32f3xx-hal/pull/98))
 - Support for `stm32f302x6` and `stm32f302x8` devices ([#132](https://github.com/stm32-rs/stm32f3xx-hal/pull/132))
 
+### Changed
+
+- Introduced auto-generated GPIO mappings based on the STM32CubeMX database
+  ([#129](https://github.com/stm32-rs/stm32f3xx-hal/pull/129))
+
 ## [v0.5.0] - 2020-07-21
 
 ### Added