Suggest DREBS to the backup section

http://dojo4.com/blog/aws-ebs-backups-we-rewrote-drebs for more info.

README.md

@@ -56,6 +56,7 @@ A curated list of amazingly awesome open source sysadmin resources inspired by [
 * [SafeKeep](http://safekeep.sourceforge.net/) - Centralized pull-based backup using `rdiff-backup`.
 * [TarSnap](https://www.tarsnap.com/) - Secure backup service with an open-source client.
 * [UrBackup](http://www.urbackup.org/) - Another client-server backup system.
+* [DREBS](https://github.com/dojo4/drebs) - AWS EBS backup script that supports strategies.
 
 ## Cloning
 *Cloning software.*