Give concrete guidance on how to extend qlog

[rmarx]

With #417 and the additional-schema proposal, we're moving to a good state for qlog's extensibility.

However, we feel additional prose is needed to make people make good use of this extensibility. Without it, we will mainly have the proper CDDL structures and some scattered prose, but no clear flowcharts/lists or similar.

For example, we can add TODOs for different use cases:

• If you want to add a fully new protocol, do xyz
• If you want to add new fields to an existing event, do abc
• If you want to add new options to an ENUM, do def

This can only be done after the extensibility stuff is properly done though, so this issue is to remind us later ;)