Documentation Improvement - REDBEAT_LOCK_TIMEOUT

REDBEAT_LOCK_TIMEOUT = CELERYBEAT_MAX_LOOP_INTERVAL * 5 needs to be explicitly set or it defaults to 25 minutes. In production this can cause a hang if the service crashes or has another issue where it does not clean itself up.

Appears to be related to this discussion: #50

Easiest if this were referenced in documents as a suggested config to avoid issues.