Add Cron Output Section

Many of us have been bit by cron's silent failures, only to find out that our jobs haven't been running for hours, or worse, days. Cron's output, though great in theory, isn't as reliable or as helpful as it could be. Our team has made sure to include this common "pitfall" in our teaching sessions and thought it would serve nicely here as well. Feedback appreciated!
@spncr2 The build output here: https://travis-ci.org/opsschool/curriculum/builds/27781415#L49
specifies that the line of line 243 (here) is too short. the line should match the length of the title above it.