Add documentation regarding OpenGL on OS X
Add documentation regarding OpenGL on OS X

nstoddard/glfw-rs,derekdreery/glfw-rs,mneumann/glfw-rs,bjz/glfw-rs,servo/glfw-rs,tomaka/glfw-rs,bvssvni/glfw-rs,PistonDevelopers/glfw-rs,derekdreery/glfw-rs