DO_NOT_MERGE: review only

This section assumes you're using the MapReduce app framework.  This is a fine assumption, as we haven't really tested how all this will work with other app frameworks.  But let's say so in the introduction:

"Currently, DCE has been tested most extensively with YARN's MapReduce application framework, and is particularly well-suited to streaming MapReduce jobs.  Also, you currently cannot configure any of the Docker settings in your MapReduce job configuration, but you can specify Docker Images for the mapper, reducer, and application master containers by using the following three JVM properties respectively: