Contributing guide should be customised further

[tobyhodges]

Maintainers have recently made several judgments about the preferred style of code and text in the lesson. CONTRIBUTING.md could be updated to mention these preferences, and perhaps some of these can be propagated up to The Carpentries style guide.

Related issues/PRS:

• Code snippet docstring formatting #291
• Consistent library naming #288
• Transition to new import convention for sckit-image? #295
• use imageio for input/output tasks? #239

I recently proposed an expansion of the contributing guide for the Collaborative Lesson Development Training curriculum, with "General" and "This Lesson" subsections, which could be used as a template to follow here too.

[K-Meech]

We should also add a note that any figures contributed going forward, should put their source code in /episodes/fig/source (where the blurring animation code is)