Document requirements to add, remove, update normative spec items

Define what constitutes:

• add
• remove
• update

of normative items.

They are not written in stone but to have them serve as a sufficiently stable guideline we can refer back to.

Aside: We assume for each potential feature there is a UCR eg #9

For example, to add a normative functionality, there needs to be x number of implementations (even if it is rough.. but within the region) before it makes it into the spec. This doesn't have to happen at the WD stage but becomes more of a solid (no pun intended) requirement as the spec matures... along the lines of a CR.

To remove, there is a collective shift on focus or technology in the ecosystem eg. +TLS HTML keygen + browser cert UX is blah.. stuff breaking down that is outside of the control of the specs and individuals involved.. Update with something equivalent if possible.

To update, show that the change keeps existing functionality but simplifies the process..

See also W3C Technical Report Development Process: https://w3c.github.io/w3process/#Reports

Start with the proposed add, remove, and update examples in this issue, then read the linked W3C Technical Report Development Process. Define stable, reviewable criteria for each normative-item change, including implementation evidence and compatibility, and document the resulting guidance for future specifications.