Added 'brew install xz' to the macOS getting started documentation.
Added 'brew install xz' to the macOS getting started documentation.

This is now required by ArchiverTest in stout.
chhsia0/mesos,kaysoky/mesos,Gilbert88/mesos,jpeach/mesos,reneploetz/mesos,gsantovena/mesos,chhsia0/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,chhsia0/mesos,dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,jpeach/mesos,gsantovena/mesos,gsantovena/mesos,gsantovena/mesos,abudnik/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,chhsia0/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,dforsyth/mesos,gsantovena/mesos,dforsyth/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,jpeach/mesos,Gilbert88/mesos,dforsyth/mesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,gsantovena/mesos,chhsia0/mesos,abudnik/mesos,gsantovena/mesos,jpeach/mesos,kaysoky/mesos,jpeach/mesos,Gilbert88/mesos,chhsia0/mesos,reneploetz/mesos