The Gnu Free Documentation License seems better suited
to the documentation than the LGPL.

Also, start writing something about the C interface

Signed-off-by: Luca Fulchir <luker@fenrirproject.org>

Also add relative (optional) cmake target: "docs"
Documentation is still incomplete, needs more
work.

Signed-off-by: Luca Fulchir <luker@fenrirproject.org>