Update pnts tile format spec to include batch table

TileFormats/Points/README.md

@@ -5,6 +5,7 @@
 * Tom Fili, [@CesiumFili](https://twitter.com/CesiumFili)
 * Patrick Cozzi, [@pjcozzi](https://twitter.com/pjcozzi)
 * Dan Bagnell, [@bagnell](https://github.com/bagnell)
+* Sean Lilley, [@lilleyse](https://github.com/lilleyse)
 
 ## Overview
 
@@ -18,6 +19,18 @@ _TODO, [#22](https://github.com/AnalyticalGraphicsInc/3d-tiles/issues/22)_
 
 Positions are defined for high-precision rendering with [RTC](http://blogs.agi.com/insight3d/index.php/2008/09/03/precisions-precisions/). [#10](https://github.com/AnalyticalGraphicsInc/3d-tiles/issues/10)
 
+## Batch Table Semantics
+
+* `TILES3D_RGBA` - per-point colors, tightly packed interleaved RGBA `uint8` (32-bits per point).  Byte length: `header.pointsLength * 4`
+* `TILES3D_RGB` - per-point colors, tightly packed interleaved RGB `uint8` (24-bits per point).  Byte length: `header.pointsLength * 3`
+* `TILES3D_COLOR` - constant color for all points.  RGBA `uint8`.  Byte length: `4`
+
+If more than one color semantic is defined, the precedence order is `TILES3D_RGBA`, `TILES3D_RGB`, then `TILES3D_COLOR`.  For example, if a tile's batch table contains both `TILES3D_RGBA` and `TILES3D_COLOR` properties, the runtime would render with per-point colors using `TILES3D_RGBA`.
+
+If no color semantics are defined, the runtime is free to color points using an application-specific default color.
+
+In any case, [3D Tiles Styling](../../Styling/README.md) may be used to change the final rendered color and other visual properties at runtime.
+
 ## File Extension
 
 `.pnts`