documentation: networking: add 6lowpan documentation

This patch adds a 6lowpan.txt into the networking documentation
directory. Currently this documentation describes how the lowpan
private data of net devices will be handled.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Suggested-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>