Add description in README for  file
Add description in README for  file

Signed-off-by: He Xiaoxi <ba8e70adeaa0032b7dc76a01185f483551d4469d@gmail.com>

alyyousuf7/swarmkit,alyyousuf7/swarmkit,jlhawn/swarmkit,dperny/swarmkit-1,AliyunContainerService/swarmkit,cheyang/swarmkit,dperny/swarmkit-1,sungwonh/swarmkit,jlhawn/swarmkit,cheyang/swarmkit,AliyunContainerService/swarmkit,sungwonh/swarmkit,docker/swarmkit,docker/swarmkit