Mention python2 only support would make usage simpler

[aseques]

My fault, but I have python 3 as the default interpreter, when launching the generator I just get this message.

$ python3 gen_outline.py --collection responses file.json 
Traceback (most recent call last):
  File "gen_outline.py", line 85, in <module>
    main()
  File "gen_outline.py", line 75, in main
    outline = make_outline(args.json_file, args.each_line, args.collection)
  File "gen_outline.py", line 50, in make_outline
    key_map = gather_key_map(iterator)
  File "gen_outline.py", line 32, in gather_key_map
    for path in key_paths(d):
  File "gen_outline.py", line 9, in helper
    for k, v in x.iteritems():
AttributeError: 'dict' object has no attribute 'iteritems'

Just by launching gen_outline.py with python2 it works as expected. Just leaving it here for reference