Document policy about adding new sys props / env vars and declarative config

[jack-berg]

We've been operating under an undocumented convention in which we're:

1. Judicious about adding support for new sys props / env vars
2. When we do add them, ensure there's equivalent config capabilities in declarative config - i.e. declarative config should be a strict superset of sys props / env vars

We should document this in our CONTRIBUTING.md

[RAGHUVEERSINGH0001]

Hi, I’d like to work on this issue and update the CONTRIBUTING.md with the documented configuration conventions. Please let me know if that sounds good.