Document Best Practice considerations regarding how much to prefetch

[dennispatterson]

The question came up at the May 2020 connectathon as to how much data should you prefetch.
It seemed a good candidate for https://cds-hooks.org/best-practices/

Ideas:

• Request prefetch data for resources that your service always needs
• For data that your service doesn't always_ need, consider asking for prefetching data that you need a majority of the time.
• Ensure that you're capturing metrics on how often FHIR resource types are used by your service so you have data to factor into adjustments in your strategy.