Clarify policy on tag inclusion

linelist offers the ability for users to define their own tags but also include some tags by default.

At the moment, we are not documenting how these default tags have been determined. This raises questions about addition of new tags. For example, the vaccineff package works with linelist data and would benefit from linelist having, e.g., a `vaccination_status` tag.

We should clarify:

- how the original default tags have been chosen
- what is the policy and the process to propose and add new tags

No files, tests, or entry points are named. Start by locating the existing documentation for default tags and reviewing how the original tags were selected; check the issue discussion for context before drafting. Done means documenting both the selection rationale and a clear process for proposing and adding tags.

Written by the indexing model from the issue text.