update installation instructions

[combatopera]

• address issues seen in problem launching Concern #30, including but not limited to avoiding pip2/python2, avoiding having to build wheels (suspect pip not recent enough), also see https://github.com/combatopera/pyven/issues/3
  • how to install gnu screen and anything else required/optional, also see portaudio and resid should not be required for install pym2149#75
  • clearer separation between the backends and how to configure
• consider adding the option of installing to a venv
• instructions should be generic and applicable to all deployables - spinoff to https://github.com/combatopera/combatopera/issues/3

[combatopera]

wheels issue is because pypi doesn't accept arch-specific wheels so i don't upload them, solution will be to make manylinux wheels