spdx.py: Add annotation to relationship

Having annotations on relationship can provide additional information
about the relationship such as how it was derived.

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit d98585aa)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>