Added crontab cheat sheet goodie

**What does your instant answer do?**
It displays some examples of how to write crontab entries.

**What problem does your instant answer solve?**
Cron is useful, but most people don't edit the crontab often enough to memorize the syntax. An instant cheat sheet is useful for that quick reminder.

**What is the data source for your instant answer?**
[`man 5 crontab`](http://linux.die.net/man/5/crontab)

**Are there any other alternative (better) data sources?**
I just came up with a few examples, which are probably not optimal. I didn't find a "canonical" cheat sheet already out there on the internet that would be brief enough to work as an instant answer.

**What are some example queries that trigger this instant answer?**
`cron help`, `crontab example`, `cron cheat sheet`

**Which communities will this instant answer be especially useful for?**
Linux sysadmins and programmers.

**Are you having any problems? Do you need our help with anything?**
Hopefully this all works - I don't completely know what I'm doing. How should I test it?