refs #6661 added change of HTTP 400 tracker api to changelog

CHANGELOG.md

@@ -4,6 +4,9 @@ This is a changelog for Piwik platform developers. All changes for our HTTP API'
 
 ## Piwik 2.10.0
 
+### Breaking Changes
+* The HTTP Tracker API does now respond with a HTTP 400 instead of a HTTP 500 in case an invalid `siteId` or `forcedVisitorId` is used.
+
 ### New APIs
 * New URL parameter `send_image=0` in the [HTTP Tracking API](http://developer.piwik.org/api-reference/tracking-api) to receive a HTTP 204 response code instead of a GIF image. This improves performance and can fix errors if images are not allowed to be obtained directly (eg Chrome Apps).