Add notes on pruning frequencies

[pronoiac]

Someone just asked about suggested pruning frequencies, and I threw out suggestions:

• right after running the backup. Tack "&& /usr/local/bin/tarsnap-prune.sh" onto the end of the lines that run the backup.
• run it manually, whenever you have the inclination; weekly or monthly. You can sanity-check the results if you haven't run it before or if you're paranoid.