Makefile: document the add-on image's arch reality (aarch64) + the multi-arch path

The add-on image is built natively on the build host (the multi-stage
Dockerfile compiles sm121 in-container): on the Mac / a DGX node that is
aarch64 — exactly the HA-on-Pi target arch. x86_64 support is a future
buildx multi-arch push (documented; needs qemu binfmt).

Makefile

@@ -45,6 +45,12 @@ addon-image:
 
 # Push the add-on image to the [registry] base (the HA add-on's config.yaml
 # references it). Requires `docker login ghcr.io`.
+# NOTE: single-arch (the build host's — aarch64 on the Mac / DGX nodes,
+# matching the Pi's arch). For a multi-arch manifest (x86_64 too) use:
+#   docker buildx build --platform linux/aarch64,linux/x86_64 \
+#     -t ghcr.io/kilork/sm121-addon:latest --push -f docker-images/sm121-addon/Dockerfile .
+# (needs qemu binfmt registered on the builder; the in-container Rust
+# compile runs under emulation for the foreign arch — slow but works).
 addon-push:
 	docker tag sm121-addon:latest ghcr.io/kilork/sm121-addon:latest
 	docker push ghcr.io/kilork/sm121-addon:latest