note important information about intervals

[opyate]

As per @mojeto #163 (comment)

I feel that this would clear up some issues around rq-scheduler usage, where folks would set a < 1 minute job interval, but it still running once a minute, and being confusing. Possible also issues with seeming "drift"?

Perhaps a maintainer can give good reasons for the following directives, then I can amend the doc:

• the job interval has to be a multiple of the scheduler interval
• the job interval has to be greater than the scheduler interval