Add Kotlin contracts to exposed Kotlin API

Author: awelless

documentation/src/docs/asciidoc/release-notes/release-notes-5.12.0-M1.adoc

@@ -49,6 +49,7 @@ JUnit repository on GitHub.
     report is now written by open-test-reporting when added to the classpath/module path
   - Information about published files is now included as attachments.
 
+* Introduced kotlin contracts for kotlin assertion methods.
 
 [[release-notes-5.12.0-M1-junit-jupiter]]
 === JUnit Jupiter