Improve Requirements docs/installation #11
Description
Currently requirements are mentioned in README, but only include instructions for individual installs. That's partly intentional because users are expected to have already started using opencv before using this library, and there are several different ways of installing that, but particularly since mss is now an available backend it would be useful to have a few requirements installation options, e.g.
[all]->numpy>=1.19.0 opencv-python>=4.2.0 mss>=3.0.0[no-screen]->numpy>=1.19.0 opencv-python>=4.2.0