Added section for BitTorrent Sync

Hi - from what I understand about redecentralize.org, BitTorrent Sync, the tool I've added to the README, is likely to be of use to folks. I'm not affiliated with BitTorrent in any way, but I've been using Sync for some weeks now on several machines (Windows and Linux boxes, located in Ireland and the UK) and have used it to reliably transfer gigabytes of data. It's a winner in my book and deserves to be more widely used.

In other news, this is my very first GitHub pull request!

cheers
Ben