Add comments for spdx/validation.go

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Add more tests and comments for validation.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Initial version of cli validator

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>