clarify arch default user in README.md (fixes #130)

5 jobs for vorburger-master-patch-80873 in 28 minutes and 39 seconds (queued for 1 second)
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  Lint
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passed shfmt #14599

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  Build
passed build #14600
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  Test
passed test-cloudimg-qemu #14602

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passed test-vagrant-boxes-format #14601

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