To add to contributing guidelines

[lwjohnst86]

From: https://github.com/merely-useful/r-rse/pull/65#pullrequestreview-621475799

• [x] minimize our attempts to accommodate instructor preference and specifically say the exercises are generally about kenyaweather
• [x] Callout boxes for solo learners
• [x] Not use master, use main when describing branch.
• [x] Voice used #61