Improve instructions for jesque Cron Jobs

[edwardotis]

Many basic cron expressions fail. For example, run every minute: '* * * * *'

I googled enough to find out that the Quartz cron expression syntax is not the same as standard unix cron expression. This is not obvious to any experienced *nix user who isn't already familiar with Quartz quirks.

It would be helpful if the instructions on the Readme (and the especially the example cron job code further down the Readme) reflected this explicitly and pointed users to the special Quartz cron expression rules page. Perhaps with a warning that they cannot use unix cron syntax in the cron expression field.