Add guidance about using id's (in Bundles) to FHIR IG

The use of id's, especially in Bundles, and their relationship to fullUrl's, is the source of quite some confusion, partially because it is somewhat scattered throughout the core FHIR documentation. This could be explained better and more concise in our FHIR IG, for the following situations:

• sending resources:
  • individually: no id expected
  • in Bundles:
    • no id expected
    • oid or uuid fullUrl expected
    • internal references based on fullUrl
• searching resources (returns a Bundle):
  • id and fullUrl expected
    • with read support:
      • RESTful absolute fullUrl required, should match id
      • internal references preferably based on relative URL
    • without read support:
      • UUID or OID based fullUrl and matching id expected. fullUrl should include the scheme, id should exclude it.
      • internal references based on fullUrl.
• reading resources:
  • id expected