Add policy for adding a new feature, functionality or modules to LocalGov Drupal

[finnlewis]

We made this decision a while ago, see:

See https://docs.google.com/document/d/1OCs3W7WFGtDNpL0U-6lhT6pQpOf2RydjtpkAMOrUeI4/edit#heading=h.56kj0gjrk8s8

It would be good to add this to the governance section of the docs.

https://docs.localgovdrupal.org/governance/