Link to detail on LFRic naming conventions

[jfrost-mo]

What problem does your feature request solve?

To help new users we should provide a link/indication of what LFRic naming conventions are at point of use within rose edit, on the lists of variables to process.

Describe the solution you'd like

Our current definition is based on the _stash_to_lfric.py file, which isn't that human readable and only contains limited information. It would be good to link to a more authoritative source, however I'm not sure if one exists.

Describe alternatives you've considered

Not sure of anywhere myself other than the spreadsheet and the STASH2LFRic file. Might be worth checking with someone from RAD. If we can't find anything I am happy for the conversations to be marked as resolved provided we open an issue so that it gets put in place for solving it when we know more.
- @daflack in #1055 (comment)

Also unsure, suspect its local information stored by Ian Boutle/Paul Earnshaw et al., as LFRic is still mostly being tested
- @jwarner8 in #1055 (comment)