Document PowerPC CI setup
We have three POWER servers running in little-endian mode at OSUOSL one owned by GHC-HQ and two (one POWER8 and one POWER9) by @trommler. The former is running Fedora and the latter are running openSUSE Leap 15.4.
@trommler will add some PowerPC big-endian machines running on vintage PowerMacs.
Some documentation exists in trommler/ci-images> (see wiki)