Documented Python 3.6 support

[impredicative]

I would like to request official Python 3.6 support, as defined in setup.py and visible at https://pypi.python.org/pypi/docker/ under Categories.

[ajomadlabs]

@shin- @impredicative Can I take up this issue

[ajomadlabs]

@shin- Is this issue taken up by someone, if not can I take this

[shin-]

#1863