Markdown tutorial

[brian-rose]

One section in the Foundations outline that is still blank: Formatted text in the notebook with Markdown

I still think this is important enough to cover, as it's a foundational skill for both working with notebooks and collaborating on GitHub.

If anyone has any useful source material for this, drop some links here!

[brian-rose]

As discussed at today's EWG meeting, some great source material from Unidata is here