Provide docs about usage of celery beat with Saleor

[korycins]

Finding the proper way of running the celery beat can be problematic, as we have tasks with custom schedulers (like handle-promotion-toggle).

In my case using PersistentScheduler caused that some tasks were called only once or twice. Using beat with django_celery_beat.schedulers:DatabaseScheduler caused the task with the custom scheduler were skipped and never called.

In Saleor code, we have our own DatabaseScheduler: scheduler saleor.schedulers.schedulers.DatabaseScheduler. AFAIK this one should be recommended to use.

Running beat with command like below:
celery --app saleor.celeryconf:app beat --scheduler saleor.schedulers.schedulers.DatabaseScheduler

will turn on the proper scheduler that is able to handle tasks like handle-promotion-toggle