add a note for the location of dtbs

I just got a BeagleBone Black revision C with debian pre-installed. it mounts as a usb drive to the host and the only way i found to change the .dtb file was to overwrite it from the host system.

BTW, reading about bts (which i never heard about before) i noticed that maybe you could instead of the dtb provide a dtbo, an overlay, which would not replace the original dtb but add things. maybe easier to maintain and keeps all other hardware alive. this repository however seems to not contain the source file .dts