Update README.md with python3 and pyobjc requirements

[coolharsh55]

Updates the system requirements and installtion / getting started sections with a note on macOS with python3 as default requiring installation of pyobj via pip for the tool to work.

[Kalmat]

Hi! Very good idea to update the docs, thank you!

I would also suggest to add a requirements.txt file in order to ease installation of all dependencies, just using python -m pip -r requirements.txt.

This is, nevertheless, the legacy way, since a most modern way to accomplish this is using setup.py, but it's fair enough in my opinion.

[coolharsh55]

IMHO it would be better to make this a pip package itself so dependencies etc. are tracked and managed canonically rather than manually rather than using setup.py packaging.python.org

[Kalmat]

You're absolutely right. I proposed that to the authors, but unfortunately they have no time to do it. We will patiently wait.

Mi suggestion about building a requirements.txt file is just a quick and dirty solution in order to ease installation.