Document procedure for adding new roles #457

I think it would be useful to modify this policy document about adding new people into existing roles to also document the policy for adding new roles (which is VERY similar).

https://github.com/astropy/astropy-project/blob/main/policies/adding-new-role-members.md

@astropy/coordinators