Update changelog

Author: teskje

CHANGELOG.md

@@ -15,6 +15,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Support for the onboard real-time clock (RTC) ([#136](https://github.com/stm32-rs/stm32f3xx-hal/pull/136))
 - Enable DMA for USART on `stm32f302` devices ([#139](https://github.com/stm32-rs/stm32f3xx-hal/pull/139))
 
+### 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