Document additive animation blending.

Docs/Reference.dox

@@ -1515,6 +1515,8 @@ Note that AnimationController does not by default stop non-looping animations au
 
 By default an Animation is played back by using all the available bone tracks. However an animation can be only partially applied by setting a start bone, see \ref AnimationState::SetStartBone "SetStartBone()". Once set, the bone tracks will be applied hierarchically starting from the start bone. For example, to apply an animation only to a bipedal character's upper body, which is typically parented to the spine bone, one could set the spine as the start bone.
 
+It is also possible to enable additive (difference) blending mode on an animation, by using \ref AnimationState::SetBlendMode "SetBlendMode()" with the ABM_ADDITIVE parameter. In this mode the AnimationState applies a difference of the animation pose to the model's base pose, instead of straightforward lerp blending. This allows an animation to be applied "on top" of the other animations, but the end result can be unpredictable in case of large difference from the base pose. Additive animations should reside on higher priority layers than lerp blended animations or otherwise the lerp blending will "blend out" the additive animation.
+
 \section SkeletalAnimation_Triggers Animation triggers
 
 Animations can be accompanied with trigger data that contains timestamped Variant data to be interpreted by the application. This trigger data is in XML format next to the animation file itself. When an animation contains triggers, the AnimatedModel's scene node sends the E_ANIMATIONTRIGGER event each time a trigger point is crossed. The event data contains the timestamp, the animation name, and the variant data. Triggers will fire when the animation is advanced using \ref AnimationState::AddTime "AddTime()", but not when setting the absolute animation time position.