Place for helpful links?

[gdevenyi]

Should references include a list of links? I have some in instructor's guide, but given the restructuring it should maybe move.

[rgaiacs]

@gdevenyi Can you give example of helpful links? And why not include the links on the lesson itself?

[gdevenyi]

https://github.com/swcarpentry/shell-novice/blob/gh-pages/_extras/guide.md#teaching-notes