man page: add page for skbmod action

Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Roman Mashak <mrv@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
2 files changed