Added External Containerizer documentation.

Adds a markdown document describing the ExternalContainerizer.

Review: https://reviews.apache.org/r/22169/
MESOS-2038: removed dead code from _runTask().

Review: https://reviews.apache.org/r/27567
CHANGELOG and docs/upgrades.md for 0.21.0.

Review: https://reviews.apache.org/r/27633