Updates to the miniMD README

Made some minor edits to the miniMD README from a performance
perspective including:

* mention of the utility of the --dataParIgnoreRunningTasks flag

* mention of the --fast flag

* reworded the disclaimer about performance to shift more blame from
  the Chapel compiler to the benchmark code (not that the compiler
  doesn't have issues as well, but recent experience suggests that
  there's probably more we could do without compiler changes as well
  and so I wanted to emphasize that performance wasn't a primary
  driver in the current implementation of the benchmark).




git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@22111 3a8e244f-b0f2-452b-bcba-4c88e055c3ca