Add 'Avoid the usage of class instance variables'

[bdewater]

Per the discussion on Slack earlier today, where we reviewed internal code where seemingly the authors were not aware of the self << class and used class instance variables instead of instance variables.

[gmalette]

To be candid, I don't think this directive belongs in a style guide. It's a software development issue, not a style one.

[bdewater]

I'm back from some time off :) @volmer thanks for your suggestion, I've trimmed the fat.