Added alert changes to the change log

Author: sxg

CHANGELOG.md

@@ -6,6 +6,9 @@ ForecastIO adheres to [Semantic Versioning](http://semver.org/).
 
 Released on March 30, 2018.
 
+### Added
+- `time`, `regions`, and `severity` properties on `Alert`
+
 ### Updated
 - `Float` properties are now `Double` on `DataPoint` and `Forecast`