Add Qt requirements to README.md according to operating system

[superbonaci]

Error after basic install:

$ python json_viewer.py
Traceback (most recent call last):
  File "json_viewer.py", line 16, in <module>
    from PyQt5 import QtCore
ModuleNotFoundError: No module named 'PyQt5'

For example for Arch required package is python-pyqt5