update the CHANGELOG

Author: japaric

cortex-m-rt/CHANGELOG.md

@@ -275,7 +275,8 @@ section                size        addr
 
 Initial release
 
-[Unreleased]: https://github.com/japaric/cortex-m-rt/compare/v0.3.15...HEAD
+[Unreleased]: https://github.com/japaric/cortex-m-rt/compare/v0.4.0...HEAD
+[v0.4.0]: https://github.com/japaric/cortex-m-rt/compare/v0.3.15...v0.4.0
 [v0.3.15]: https://github.com/japaric/cortex-m-rt/compare/v0.3.14...v0.3.15
 [v0.3.14]: https://github.com/japaric/cortex-m-rt/compare/v0.3.13...v0.3.14
 [v0.3.13]: https://github.com/japaric/cortex-m-rt/compare/v0.3.12...v0.3.13