Update CHANGELOG.md

Add changelog block for next release, expected to be 4.6

Author: aymanhab

CHANGELOG.md

@@ -6,6 +6,10 @@ request related to the change, then we may provide the commit.
 
 This is not a comprehensive list of changes but rather a hand-curated collection of the more notable ones. For a comprehensive history, see the [OpenSim Core GitHub repo](https://github.com/opensim-org/opensim-core).
 
+v4.6
+=====
+
+
 v4.5.2
 ======
 - The performance of `getStateVariableValue`, `getStateVariableDerivativeValue`, and `getModelingOption` was improved in