Add rufus scheduler `every` notice to readme

Currently, when you set up a job using `every` syntax - it starts calculating the schedule time from the moment of deploy. So, in case you want to set up a task with schedule time being `every 30h`, and you redeploy your application in 15 hours from that moment - it will start calculating from zero again. If you deploy once-twice per day, and set tasks like, say, `every 20h` - it will be rarely getting run, if run at all.
Unfortunately, it's not stated in the README.md.
This commit adds it to readme so that new users don't get into this trouble cause this feature is a bit unobvious.