add documentation for new option to set install area
add documentation for new option to set install area

skosukhin/spack,tmerrick1/spack,lgarren/spack,skosukhin/spack,tmerrick1/spack,lgarren/spack,lgarren/spack,krafczyk/spack,mfherbst/spack,mfherbst/spack,krafczyk/spack,tmerrick1/spack,iulian787/spack,skosukhin/spack,iulian787/spack,TheTimmy/spack,LLNL/spack,LLNL/spack,matthiasdiener/spack,LLNL/spack,matthiasdiener/spack,krafczyk/spack,TheTimmy/spack,LLNL/spack,EmreAtes/spack,matthiasdiener/spack,matthiasdiener/spack,lgarren/spack,lgarren/spack,EmreAtes/spack,TheTimmy/spack,iulian787/spack,krafczyk/spack,mfherbst/spack,mfherbst/spack,TheTimmy/spack,matthiasdiener/spack,tmerrick1/spack,skosukhin/spack,EmreAtes/spack,tmerrick1/spack,EmreAtes/spack,EmreAtes/spack,TheTimmy/spack,mfherbst/spack,skosukhin/spack,krafczyk/spack,LLNL/spack,iulian787/spack,iulian787/spack