Add security considerations for IANA registration

[jordan2175]

Here is the list of items that were discussed on our last working call

----- SNIP TODO LIST -----

• Reference to STIX and other content
• Authentication/Authorization
• Confidentiality and TLS
• HTTP Basic security considerations
• Unicode
• Errors - descriptive text leaking info
• Errors - return code for things you don't have access to (leak info about existence of something, implementation of something, vs. just not having access)
• Reference HTTP/HTTPS security considerations (maybe need to mention URL traversals in general TAXII security considerations but not for this IANA media type section)

[jordan2175]

Done