Clarify policy on tag inclusion

[Bisaloo]

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