all: add a local copy of the license (FDL v1.3)

The license information is take from the initial import of the
book from Richard Haines:

  Copyright © 2020 Richard Haines.

  Permission is granted to copy, distribute and/or modify this
  document under the terms of the GNU Free Documentation License,
  Version 1.3 or any later version published by the Free Software
  Foundation.

  See: http://www.gnu.org/licenses/fdl-1.3.html

Signed-off-by: Paul Moore <[email protected]>

Author: pcmoore