Add Backupninja backup tool

README.md

@@ -48,6 +48,7 @@ A curated list of amazingly awesome open source sysadmin resources inspired by [
 
 * [Amanda](http://www.amanda.org/) - Client-server model backup tool.
 * [Bacula](http://www.bacula.org) - Another Client-server model backup tool.
+* [Backupninja](https://labs.riseup.net/code/projects/backupninja) - Lightweight, extensible meta-backup system.
 * [Backuppc](http://backuppc.sourceforge.net/) - Client-server model backup tool with file pooling scheme.
 * [Burp](http://burp.grke.org/) - Network backup and restore program.
 * [Rsnapshot](http://www.rsnapshot.org/) - Filesystem Snapshotting Utility.