add python 3 support note
add python 3 support note

python-fedex-devs/python-fedex,gtaylor/python-fedex,gtaylor/python-fedex,python-fedex-devs/python-fedex,python-fedex-devs/python-fedex,gtaylor/python-fedex,python-fedex-devs/python-fedex,gtaylor/python-fedex