message: add doc about parallelization which refers to the stan-math wiki
diff: @@ -622,6 +622,30 @@ to force more accurate floating point computations:
  \end{itemize}
  +\section{Optional Parallelization Support}
  +
  +\CmdStan has optional support for within-chain parallelization using
  +threads or multiple processes which are linked via the message passing
  +interface (MPI).
  +%
  +\begin{description}
  + \item[Threads] can utilize multiple cores on a single
  + machine. Please refer to
  + \url{https://github.com/stan-dev/math/wiki/Threading-Support} for
  + instructions on how to setup threading.
  + \item[MPI] is typically used on large computing clusters. The MPI
  + standard allows to link together a large number of processes which
  + can run locally on the same and/or accross potentially many
  + different machines. Please refer to
  + \url{https://github.com/stan-dev/math/wiki/MPI-Parallelism} for
  + more details.
  +\end{description}
  +%
  +The within-chain parallelization is used in the Stan language to
  +parallelize the evaluation of the \Verb|map_rect| command. Note that
  +MPI parallelism will take precendence in case both features are
  +enabled.
  +
  \section{Optional Components for Developers}
  \CmdStan is developed using the following set of tools. The various
  