Add changelog entry for changed postbuild

Author: hlovdal

CHANGELOG.md

@@ -9,6 +9,11 @@ All notable changes to this project will be documented in this file.
 - Various minor updates to documentation [#1349](https://github.com/microsoft/vscode-arduino/pull/1349)
 - Don't set `commandPath` when `arduino-cli` is `true` [#1297](https://github.com/microsoft/vscode-arduino/pull/1297)
 
+### Breaking Changes
+- The `postbuild` external command will now run unconditionally after every
+build, successfull or not. Previously it only ran on successfull builds despite
+the documentation saying it should run after every build.
+
 ## Version 0.4.4
 
 - Release date: August 26, 2021