Example LaunchDaemon plist and README

I'm using a Mac Mini and attached Drobo to back up my GitHub Enterprise server. I went through the process of configuring scheduled tasks The Right Way™ with launchd (cron is deprecated on OS X) so thought it might be nice to ship an example plist and some instructions in an 'extras' directory in the repo.

Feel free to close this out and suggest it should be documentation if preferred.