Added a description of the organization of the PDL::Graphics::PLplot module as well as expanding the usage example.
Added a description of the organization of the PDL::Graphics::PLplot module as well as expanding the usage example.

svn path=/trunk/; revision=7368

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot