message: add volume texture documentation
diff: @@ -7,6 +7,8 @@ Version History
  textures, thus `OSPTexture2D` and `ospNewTexture2D` are now
  deprecated, use the new API call `ospNewTexture("texture2d")`
  instead
  + - Added new `volume` texture type to visualize volume data on arbitrary
  + geometry placed inside the volume
  - Added new framebuffer channels `OSP_FB_NORMAL` and `OSP_FB_ALBEDO`
  - Applications can get information about the progress of rendering the
  current frame, and optionally cancel it, by registering a callback
  