Improve CONTRIBUTING.md

[fmichonneau]

This is an umbrella issue to capture things that should be added/made clearer in the CONTRIBUTING file. Some of the information listed here could also go in The Carpentries handbook depending on the level of details needed. (This issue will be edited as needed).

• In the light of the Bug BBQ, making clearer when to edit _episodes or _episodes_rmd
• Being more explicit about the system dependencies and installation instructions to render the website locally (Add note on Jekyll requirement to Readme swcarpentry/python-novice-gapminder#306)

[rgieseke]

Being more explicit about the system dependencies and installation instructions to render the website locally (swcarpentry/python-novice-gapminder#306)

The setup.md might also be a good place for such instructions.

[fmichonneau]

setup.md is intended for learners and aimed at informing them on how to get their laptop setup before the workshop. Knowing how to render the site is beyond what we teach in workshops.

[rgieseke]

I see, a link in the Readme to an 'Installation' section in the CONTRIBUTING.md file is probably a good option then.