docs: contribution guidance for test setup and approach

[tdashworth]

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)

[tdashworth]

@leroy-douglas @osagiestar please share your own opinions on what would be helpful to get setup. While I don't want to remove all human interaction when onboarding someone, it would be useful to refer to some documentation.