Add description of the API to the module docstring.

This is not all implemented yet, mostly a description of what I want it to look
like in the end. This required figuring out the right matrices based on some of
the random ones in Travis' code. Should not be too difficult.

scikits/image/color/colorconv.py

@@ -3,6 +3,15 @@
 
 """Functions for converting between color spaces.
 
+The "central" color space in this module is RGB, more specifically the linear
+sRGB color space using D65 as a white-point (IEC 61966-2-1).  This represents a
+standard monitor (w/o gamma correction).
+
+The API consists of functions to convert to and from RGB as defined above, as
+well as a generic function to convert to and from any supported color space
+(which is done through RGB in most cases).
+
+
 Supported color spaces
 ----------------------
 - RGB
@@ -188,6 +197,14 @@ xyz_from_rgb =  [[0.412453, 0.357580, 0.180423],
 
 rgb_from_xyz = linalg.inv(xyz_from_rgb)
 
+# Matrices from Jain. TODO: find complete reference.
+xyz_from_rgbcie = [[0.490, 0.310, 0.200],
+                   [0.177, 0.813, 0.011],
+                   [0.000, 0.010, 0.990]]
+
+rgbcie_from_xyz = linalg.inv(xyz_from_rgbcie)
+
+
 #-------------------------------------------------------------
 # The conversion functions that make use of the matrices above
 #-------------------------------------------------------------