Document line break

[rgaiacs]

We make a decision to limit lines at 100 characters, see https://github.com/swcarpentry/lesson-example/blob/gh-pages/bin/lesson_check.py#L100. But this must be documented at http://swcarpentry.github.io/lesson-example/04-formatting/ including rules of how to setup your text editor.

Why limit the length of lines?

The main motivation is to help when translating the lesson, although translations is yet in-progress (see https://github.com/Carpentries-ES/board/). Short lines will speed up when we update the translations because conflicts will be easier to resolve.

Conflicts also happen when merging some pull requests but doesn't happen often.

Related Issues/Pull Request

swcarpentry/shell-novice#639