Small Changes to Parallel Documentation

As a follow up to Issue [#616] (https://github.com/qiime/qiime/issues/616), I am submitting a small change to the documentation to show how to modify your ``qiime_conf`` to use the  existing ``start_parallel_job`` script.  I don't know if this is the best place to put this but I  think I would have benefitted from having this spelled out so maybe others would as well.

In my case this modification allows me to run the denoising in parallel on a workstation (not a cluster).


thanks,
zach cp