C2MS — Product Messages comments

(Product Messages) Section 7.11 (Though these comments are related to guidance for custom messages

• Consider providing guidance regarding what the minimal set of attributes should be for a custom message type.
• Indicate that custom messages can be considered to become sponsored message types in future versions of the specification.
• Extending the lexicon should be encouraged by the implementation, not discouraged, as the strictest of the GMSEC 4.x MIST modes does.

Change section 8.1 to mention extensions

Change: "The Message Header is a super-class for all C2MS messages in the sense that all fields in the Message Header appear in all of the C2MS messages." to "The Message Header is a super-class for all C2MS messages and any extensions in the sense that all fields in the Message Header appear in all of the C2MS messages.