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