beaglebone-black reference url

reference :

　　https://github.com/beagleboard/beaglebone-black/wiki/System-Reference-Manual

　　https://beagleboard.org/getting-started

　　https://github.com/beagleboard/beaglebone-getting-started

　　https://www.digikey.com/eewiki/display/linuxonarm/BeagleBone+Black#BeagleBoneBlack-Ubuntu13.10

　　http://www.cnblogs.com/dylancao/p/8608138.html

       https://elinux.org/BeagleBone_Community