Add a --pid=host feature to expose the host PID space to the container

Docker 1.5.0+ introduces a --pid=host feature which allows sharing of PID
namespaces between baremetal and containers.  This is useful for atomic
upgrades, atomic rollbacks, and monitoring.

For more details of a real-life use case, check out:
http://sdake.io/2015/01/28/an-atomic-upgrade-process-for-openstack-compute-nodes/

Signed-off-by: Steven Dake <[email protected]>

docs/yml.md

@@ -264,6 +264,16 @@ net: "none"
 net: "container:[name or id]"
 net: "host"
 ```
+### pid
+
+```
+pid: "host"
+```
+
+Sets the PID mode to the host PID mode.  This turns on sharing between
+container and the host operating system the PID address space.  Containers
+launched with this flag will be able to access and manipulate other
+containers in the bare-metal machine's namespace and vise-versa.
 
 ### dns