add instructions for multi-arch build at top of Dockerfile

Dockerfile

@@ -6,6 +6,10 @@
 #     docker run -v "$PWD/data":/data archivebox add 'https://example.com'
 #     docker run -v "$PWD/data":/data -it archivebox manage createsuperuser
 #     docker run -v "$PWD/data":/data -p 8000:8000 archivebox server
+# Multi-arch build:
+#     docker buildx create --use
+#     docker buildx build . --platform=linux/amd64,linux/arm64,linux/386,linux/arm/v7 --push -t archivebox/archivebox:latest -t archivebox/archivebox:dev
+
 
 FROM python:3.10-slim-bullseye