update doc index with explicit Python version note
update doc index with explicit Python version note

people seem to still manage to not be able to find that PyZMQ works with Python 3.

Mustard-Systems-Ltd/pyzmq,yyt030/pyzmq,swn1/pyzmq,Mustard-Systems-Ltd/pyzmq,caidongyun/pyzmq,caidongyun/pyzmq,ArvinPan/pyzmq,caidongyun/pyzmq,dash-dash/pyzmq,Mustard-Systems-Ltd/pyzmq,ArvinPan/pyzmq,swn1/pyzmq,dash-dash/pyzmq,yyt030/pyzmq,ArvinPan/pyzmq,dash-dash/pyzmq,yyt030/pyzmq,swn1/pyzmq