Add changelog entry for pwm input gpio fix (#58)

Author: gbip

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Add methods `stop`, `release` and `clear_update_interrupt_flag` to `Timer` (`clear_update_interrupt_flag` does not apply to `Timer<SYST>`)
 - Add timer interrupt example using RTFM
 - Implement IndependentWatchdog for the IWDG peripheral
+- Fix gpio misconfiguration when using a timer in pwm input mode. Now the gpio has to be configured in floating input mode.
 
 ### Changed