atari_py probably doesn't work on windows

[hnc13]

Guessing with @jjfiv:

Unable to execute 'make build -C atari_py/ale_interface -j 3'. 
HINT: are you  sure `make` is installed?
 error: [WinError 2] The system cannot find the file specified
--

[jjfiv]

openai/atari-py#29
openai/atari-py#5

[jjfiv]

Promising leads: (https://stackoverflow.com/questions/42605769/openai-gym-atari-on-windows/46739299)

pip install --no-index -f https://github.com/Kojoley/atari-py/releases atari_py

[jjfiv]

This alternative pip install worked just now on Windows 10 (Feb 13, 2019) with the version of gym we need. Time to add to the README.md.

[etosch]

What's the status on this issue? FWIW travis appears to support windows: https://docs.travis-ci.com/user/reference/windows/