[BUG] Add python version restrictions to installation section of the docs

[grafuls]

No description provided.

[kedark3]

I have encountered issues installing this package with python 2.7, sample traceback snippet:

Extracting disruption_generator-0.2.0-py2.7.egg to /home/kkulkarn/git/disruption_generator/venv/lib/python2.7/site-packages
  File "/home/kkulkarn/git/disruption_generator/venv/lib/python2.7/site-packages/disruption_generator-0.2.0-py2.7.egg/disruption_generator/cli.py", line 41
    async def execute(experiments_path):
            ^
SyntaxError: invalid syntax

  File "/home/kkulkarn/git/disruption_generator/venv/lib/python2.7/site-packages/disruption_generator-0.2.0-py2.7.egg/disruption_generator/disruption_generator.py", line 7
    async def main():
            ^
SyntaxError: invalid syntax

  File "/home/kkulkarn/git/disruption_generator/venv/lib/python2.7/site-packages/disruption_generator-0.2.0-py2.7.egg/disruption_generator/listener/alistener.py", line 20
    async def run_client(self, filepath, expression, timeout):
            ^
SyntaxError: invalid syntax

  File "/home/kkulkarn/git/disruption_generator/venv/lib/python2.7/site-packages/disruption_generator-0.2.0-py2.7.egg/disruption_generator/trigger/trigger.py", line 20
    async def run_client(self, cmd):
            ^
SyntaxError: invalid syntax

[pnovotny]

@kedark3 the project is supported only for Python 3.5+.

[grafuls]

This is being addressed here: #48
Result: https://disruption-generator.readthedocs.io/en/docs/installation.html