[Docs update] guidance on deliberate use of additional fields

[odscjen]

If you are proposing changes or updates to the RDL documentation, then please fill in the below template.

What is the context or reason for the change?

Users coming from other standards may need to include fields that don't currently have a direct mapping to RDLS.

What is your proposed change?

They can do this by just adding additional fields as columns in the spreadsheet templates and the convertor will add them to the JSON it produces. But there's no guidance on when this might be acceptable and how to best do this. What we want to encourage in this regard should be decided and appropriate guidance authored.