docs: contribution guidance for test setup and approach

For new contributors, it would be good to have some context and steps to follow when testing changes. I think this should include:
- creating an environment
- installing specflow add-on
- configuring test (in-file, environment variables, runsettings) (warning not to commit!) (short explanation of each property)
- environment details
- headless
- user details
- application user
- first test run
- solution install
- mindful of test durations (don't run everything at once)