Update CHANGELOG.md

Author: wahln

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Changelog
 
-## Current Development Branch
+## Version 3.1.0 - "Cleve"
 
 ### Major Changes and New Features
 
@@ -129,4 +129,4 @@ First official release of matRad including
 - DICOM import including dose and particle pencil beam scanning plan objects
 - matRad standalone version
 - Improved GUI workflow
-- Many bug fixes and many new bugs...
+- Many bug fixes and many new bugs...