Add ability to suspend/resume a container.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)
Add documentation for suspend/resume.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)