Instructions for setting up bbb with debian + xenomai

Hej Oscar,

I added my instructions in another file, beaglebone-xenomai-debian.md and added a reference to this in README.md. This should (hopefully) make it easier for you to handle the pull request.

If you grep for "NOTE" in the new file, you'll see the places where I had some (slightly) different experiences than what your instructions in README.md would lead me to expect.