Suggestions for README improvements

[markstos]

• It could be clarified whether calculations are inclusive or exclusive of the current date of 'now'. The answer appears to be inclusive. It coudl be worth mentioning that this means that this tool should run after daily snapshotting happens.
• It could also be clarified that "days:3" does NOT mean the "the 3 most recent daily backups", it means "backups from the 3 most recent days". If you have 3 three daily backups but none in the last 3 days, this tool would select all your backups for deletion and leave you with zero. There's no problem as long as the backups are running fine. But if you are backups get stalled and your pruning script does not, you'll slowing eat through your backups.