Update release notes

Docs/ReleaseNotes.md

@@ -2,6 +2,13 @@
 
 For breaking API changes see [this document](https://github.com/jrouwe/JoltPhysics/blob/master/Docs/APIChanges.md).
 
+## Unreleased changes
+
+### Bug fixes
+
+* Fixed an issue where the bounding volume of a HeightFieldShape was not properly adjusted when calling SetHeights leading to missed collisions.
+* Workaround for CMake error 'CMake Error: No output files for WriteBuild!' when using the 'Ninja Multi-Config' generator.
+
 ## v5.1.0
 
 ### New functionality