Clarify expectations for caching discovery responses

[jmandel]

Reported by @carloseberhardt via http://wiki.siframework.org/BlueButtonPlus+Pull+API+Documentation+Consensus

App registry appears to be out of scope for this. I'm not sure it belongs in the PULL api spec.

If we must force developers to do the discovery dance, there needs to be some expectation about how long they can cache the responses. Requiring a call every time to discover the endpoints is unwieldy and inefficient. Should be clarified in the spec, IMO.

[jricher]

All of the discovery endpoints should explicitly use existing HTTP caching mechanisms.

[jmandel]

@carloseberhardt is that sufficient clarification? Should we include examples in the spec?