Adding documentation for '--cgroup-parent' option.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)

docs/sources/reference/commandline/cli.md

@@ -805,6 +805,7 @@ Creates a new container.
       -c, --cpu-shares=0         CPU shares (relative weight)
       --cap-add=[]               Add Linux capabilities
       --cap-drop=[]              Drop Linux capabilities
+      --cgroup-parent=           Optional parent cgroup for the container
       --cidfile=""               Write the container ID to the file
       --cpuset-cpus=""           CPUs in which to allow execution (0-3, 0,1)
       --device=[]                Add a host device to the container