Update changelog

Author: nickbianco

CHANGELOG.md

@@ -65,6 +65,9 @@ v4.6
     decorations were emitted for both `true` and `false`, resulting in double-emission).
   - It will now check for NaNed vectors coming from the underlying expression, skipping emission
     if one is detected (previously: it would emit decorations with `NaN`ed transforms).
+- Added the property `activation_dynamics_smoothing` to `DeGrooteFregly2016Muscle`. This
+property uses the model's original value of 0.1 as a default, but users may consider
+increasing this value (e.g., 10.0) so that the activation and deactivation speeds of the model better match the activation and deactivation time constants.
 
 v4.5.1
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