Howtogeek posted How to Install and Dual Boot Linux on a Mac
A quote from the article:
Installing Windows on your Mac is easy with Boot Camp, but Boot Camp wont help you install Linux. Youll have to get your hands a bit dirtier to install and dual-boot a Linux distribution like Ubuntu. If you just want to try Linux on your Mac, you can boot from a live CD or USB drive. Insert the live Linux media, restart your Mac, press and hold the Option key, and select the Linux media on the Startup Manager screen. We installed Ubuntu 14.04 LTS to test this process. Install rEFInd rEFInd is a boot manager that will allow you to choose between Mac OS X, Linux, Windows, and other operating systems when you boot your computer.How to Install and Dual Boot Linux on a Mac @ Howtogeek