Added doc comment on ColorFormat.Empty.

Source/OpenTK/Graphics/ColorFormat.cs

@@ -130,6 +130,9 @@ namespace OpenTK.Graphics
         /// <summary>Gets the sum of Red, Green, Blue and Alpha bits per pixel.</summary>
         public int BitsPerPixel { get { return bitsPerPixel; } private set { bitsPerPixel = value; } }
 
+        /// <summary>
+        /// Defines an empty ColorFormat, where all properties are set to zero.
+        /// </summary>
         public static readonly ColorFormat Empty = new ColorFormat(0);
 
         #endregion