Added note about opengl and vispy

doc/source/3dgraphics/index.rst

@@ -5,7 +5,10 @@ The 3D graphics system in pyqtgraph is composed of a :class:`view widget <pyqtgr
 several graphics items (all subclasses of :class:`GLGraphicsItem <pyqtgraph.opengl.GLGraphicsItem>`) which 
 can be added to a view widget.
 
-**Note:** use of this system requires python-opengl bindings. Linux users should install the python-opengl
+**Note 1:** pyqtgraph.opengl is based on the deprecated OpenGL fixed-function pipeline. Although it is
+currently a functioning system, it is likely to be superceded in the future by `VisPy <http://vispy.org>`_.
+
+**Note 2:** use of this system requires python-opengl bindings. Linux users should install the python-opengl
 packages from their distribution. Windows/OSX users can download from `<http://pyopengl.sourceforge.net>`_.
 
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