Added documentation on how to build ITK
Added documentation on how to build ITK

ITK for the tomviz superbuild is generated by building ITK and then
archving the result to save on build time.  This file explains how to
create the zip/tar.gz archives that are used in the build in the place of
building ITK.

mathstuf/tomviz-superbuild,cjh1/tomviz-superbuild,OpenChemistry/tomviz-superbuild,mathturtle/tomviz-superbuild,OpenChemistry/tomviz-superbuild,mathstuf/tomviz-superbuild,cjh1/tomviz-superbuild,mathturtle/tomviz-superbuild