Add changelog entry for itoa

Author: hlovdal

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 ### Added
 
+- Provide an `itoa` function. It is present in Arduino's runtime environment but not on most (all?) host systems because itoa is not a portable standard function.
+
 ### Changed
 
 ### Deprecated