add a short entry on how to use the whenever gem to schedule cron jobs

[jmartin-sul]

this has become standard DLSS practice for setting up cron jobs on servers, because it allows versionable and automatically deployable (via cap integration) setup of the cron jobs we want to run on our VMs.

this approach ends up being more maintainable and traceable in the long run than creating cron jobs by hand on the VMs, but it's also an extra level of indirection, and may be unfamiliar for ops people who are used to just creating cron jobs by hand.

one gotcha: as the cron job definitions change over time, occasionally whenever fails to clear defunct cron jobs (which then must be deleted by hand). a bit of a rough edge, tolerable, but also worth documenting.

other bonuses: the DSL used by whenever for declaring cron jobs is easier to read than the plain cron declarations, integrates more tightly with the ruby code cron would usually be calling anyway, and is OS independent.

https://github.com/javan/whenever

[jmartin-sul]

addendum from @tallenaz:

I wonder if it might be good to also talk about how we can also manage crons via puppet, and maybe try to document something about use cases for those two approaches

i agree, that'd be a good thing to add too.