Update contributing guide

Follows up on discussion starting at https://unimelbrcs.slack.com/archives/G0107A3R6AG/p1588207939177500

While PEP8 standards make (at least some) sense for source code, they have questionable merit for markdown. This change makes several changes to contribution guidelines for pure content/markdown. It also rearranges and consolidates guideline topics for a more orderly flow.