Usage notes in the doc

Based on a pull request by weizhou <weizhou@redhat.com> which edited the
README directly instead of the POD.

weizhou's text was rewritten slightly.
Releng 0.11
Don't try to delete already deleted tickets
Auto-delete threshold
Use the first active status instead of hardcoding "open"

More flexibility for lifecycles.
Quick lightweight tool to gather spam report stats
Tool to apply current auto-delete threshold retroactively

Use with caution!
Releng 0.12
Enrich README for detail usage.