Add additional guidance about population of meta.profile

Add additional text to section 5.5 Profile use and declaration of the main IG: The implementation guides for the various information standards list the profiles to use for the FHIR resources. A profile is a statement from the originator about the rules that this resource was created against. In the interest of interoperability it is important that, when using said information standard: Each resource SHALL be a valid instance of the applicable profile. Note that this applies not only to profiles listed directly in the IG, but also to profiles referred from/used by listed profiles. Each resource SHALL include the canonical URL of this profile in the  meta.profile  element. The canonical of compatible base or derived profiles MAY be included as well. A receiving system MAY use the stated value of meta.profile for validation, documentation or other purposes. A receiving system MAY use other profile(s) than stated in meta.profile, e.g. when it has derived, more constrained profiles based on the stated meta.profile. Reasons for additional validation may include protecting the integrity of the receiving system or determination of internal process flow.