Elaborate on skinning weights sum

specification/2.0/README.md

@@ -935,12 +935,16 @@ The mesh for a skin is defined with vertex attributes that are used in skinning
 }
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-The number of joints that influence one vertex is limited to 4 per set, so referenced accessors must have `VEC4` type and following component formats:
+The number of joints that influence one vertex is limited to 4 per set, so referenced accessors must have `VEC4` type and following component types:
 
 * **`JOINTS_0`**: `UNSIGNED_BYTE` or `UNSIGNED_SHORT`
 * **`WEIGHTS_0`**: `FLOAT`, or normalized `UNSIGNED_BYTE`, or normalized `UNSIGNED_SHORT`
 
-The joint weights for each vertex must be non-negative, and normalized to have a linear sum of `1.0`. No joint may have more than one non-zero weight for a given vertex.
+The joint weights for each vertex must be non-negative. No joint may have more than one non-zero weight for a given vertex.
+
+When the weights are stored using `FLOAT` component type, their linear sum must be as close as possible to `1.0` for a given vertex. When the weights are stored using `UNSIGNED_BYTE` or `UNSIGNED_SHORT` component types, their linear sum before normalization must be `255` or `65535` respectively.
+
+> **Implementation Note:** Otherwise, vertices would be deformed very significantly because the weight error would get multiplied by the joint position. So an error of `1/255` in the weight sum is actually unacceptably large.
 
 In the event that of any of the vertices are influenced by more than four joints, the additional joint and weight information will be found in subsequent sets. For example `JOINTS_1` and `WEIGHTS_1` if present will reference the accessor for up to 4 additional joints that influence the vertices. Note that client implementations are only required to support a single set of up to four weights and joints, however not supporting all weight and joint sets present in the file may have an impact on the model's animation.