Add note on skinning evaluation.

doc/skinning.md

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+# Skinnig in usdSkel
+
+https://graphics.pixar.com/usd/dev/api/_usd_skel__intro.html
+
+## Evaluation of skinning
+
+Terminology
+
+* bindTransforms : World matrices of each joint
+* restTransforms : Local matrices of each joint. Need to evaluate in bone(joint) hierarchy. Used when there is no corresponding transform from `SkelAnimation` 
+* SkelAnimation : (local?) matrices for each joint
+* Sparse : When SkelAnimation supplies(maps?) animation transform to subset of joints
+* Non-Sparse : When SkelAnimation supplies(maps?) animation transform to all of joints
+
+`restTransforms` could be optional when `SkelAnimation` supply transforms  for all joints, but it seems usdview and Houdini does not allow omitting `restTransforms`(`restTransforms` is not automatically calculated from `bindTransforms`)
+
+Also, without `bindTransforms` skinning does not work well(it is not automatically caculuated from `restTransforms`) 
+
+So, both `bindTransforms` and `restTransforms` must exist in USD.
+
+### Skinning matrix 
+
+
+```
+skinM = inv(jointWorldSpaceBindTransform) x jointSkelSpaceTransform
+
+jointSkelSpaceTransform = jointLocalSpaceTransform * parentJointSkelSpaceTransform
+
+//
+
+jointWorldSpaceTransform = jointLocalSpaceTransform *
+    parentJointSkelSpaceTransform * skelLocalToWorldTransform
+
+```
+
+### Skinning a point
+
+```
+# geomBindTransform = `primvars:skel:geomBindTransform` 
+
+skelSpacePoint = geomBindTransform.Transform(localSpacePoint)
+p = (0,0,0)
+for jointIndex,jointWeight in jointInfluencesForPoint:
+    p += skinningTransforms[jointIndex].Transform(skelSpacePoint)*jointWeight
+```
+
+
+EoL.