add spec for exec a new process inside a container

document desired execin process behavior for v1 container specification

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
fix some grammar errors

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
move cgroup requirements to planned additions

also document that the executor will only try to remove all the children in a
best-effort way.

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
Merge pull request #290 from dqminh/exec-spec

add spec for exec a new process inside a container