release-notes: note ability to build powerpc64le-linux NixOS ISOs

This commit adds a mention to the release notes of the fact that
NixOS 23.05 can build installer ISOs for a new platform.

nixos/doc/manual/release-notes/rl-2305.section.md

@@ -374,6 +374,8 @@ In addition to numerous new and upgraded packages, this release has the followin
 
 - The minimal ISO image now uses the `nixos/modules/profiles/minimal.nix` profile.
 
+- NixOS installer ISOs can now be built for `powerpc64le-linux`; see `nixos/modules/installer/sd-card/sd-image-powerpc64le.nix` and [PR 192672](https://github.com/NixOS/nixpkgs/pull/192672).  Hydra does not support this platform, so you must build the binaries yourself.
+
 - The `ghcWithPackages` and `ghcWithHoogle` wrappers will now also symlink GHC's
   and all included libraries' documentation to `$out/share/doc` for convenience.
   If undesired, the old behavior can be restored by overriding the builders with