From the call, there are several changes needed on this resource for creating referrals/exercise prescriptions:

1. We should include a 'Notes' element to allow conveying additional details (e.g. appropriate time and mechanism for service provider to contact patient).  Should indicate that Service Providers typically won't see notes that are added once a ServiceRequest has been accepted though.
2. Status should be constrained to prohibit 'unknown', as that's not appropriate in our use-case
3. authoredOn and requester should be 1..1 - these will always be known and are important for payment
4. For the same reason, there should be an invariant requiring at least one of reasonCode and reasonReference