undocumented how to enable pid namespace isolation and shared filesystem isolation

http://mesos.apache.org/documentation/latest/mesos-containerizer/ doesn't actually mention how to enable shared filesystem- and pid namespace isolation.

I'll suggest adding something like:
"To enable the Shared Filesystem isolator, append filesystem/shared to the --isolation flag when starting the slave."

.. and:
"To enable the Pid Namespace isolator, append namespaces/pid to the --isolation flag when starting the slave."