include/list -arm64 builds in README.md?

[yarikoptic]

those are present on https://quay.io/repository/singularity/singularity?tab=tags but not listed in README.md

NB my motivation -- I was looking for how they are built... I guess quay.io allows to build for different architectures from the same Dockerfile?

[vsoch]

@yarikoptic just look at a version branch and you'll see two Dockerfile: https://github.com/singularityhub/singularity-docker/tree/v3.10.0