add note on Python 2 support in install instructions

modules/doc/install.rst

@@ -57,6 +57,9 @@ If you would like to use the :mod:`molecular mechanics <ost.mol.mm>` module:
 
 * `OpenMM <https://simtk.org/home/openmm>`_ (6.1)
 
+We do not provide backwards compatibility to Python 2.7. The last
+release supporting Python 2.7 is 1.11.0.
+
 
 Getting the Source Code
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