Add appendices for media type and URI scheme registrations.

[ghost]

Add appendices for the IANA registrations of

• Media type application/sarif+json
• Media type application/sarif-external-properties+json (not yet registered at the time of this writing)
• URI scheme sarif (not yet registered at the time of this writing)

Per Chet Ensign of OASIS:

Generally, the way TCs do this is to include the registration as part of the specification, usually in an appendix.

Unfortunately we didn't complete the registrations before publishing the first version of the spec. So if there's ever another version, we should do this.

@michaelcfanning FYI

[dmk42]

Now that the Errata document has been published, this has been fixed in the official version of the standard.