Improve developer documentation

[ehossack]

This could be as simple as updating the CONTRIBUTING.md to be more accessible:

• start with recommended system installation of python(s) first before other sections
• link to the configuration of the commands run on CI build + expectations on what is done locally vs. at pr-time
• high-level overview of repo structure and test strategy (old strategy vs. new, where to add tests and when)
• the need for a CHANGELOG message

Also I'd recommend trying a fresh setup by literally following the instructions, it's quite sparse, and as I said, I cannot get python <= 3.6 functioning on my machine, so others running a similar later version of macOS 11 are likely to experience the same issue.

[ppolewicz]

This sounds like a great idea. Would you care to adjust the documentation as outlined above, provide first draft at least? I'd help refine it, if you'd help start it up