1) I updated the build and install notes for 1.8.1.

2) I modified the pengra config site file to use the 4.6.1 install of
   Silo so that the Silo reader would be able to read HDF5 based Silo
   files. This resolves visit00008532.



git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@3423 18c085ea-50e0-402c-830e-de6fd14e8384