docs(README): add a hint about multi-arch builds #6404

Job	Run time
flavor	4s
bake-annotations	27s
bake	13s
docker-push	32s
global-exps	4s
images	5s
json	3s
labels	5s
multi-images	5s
no-images	5s
output-env	14s
sep-tags ( )	7s
tag-match (\d.\d.\d, 0)	5s
sep-tags (,)	7s
tag-semver (auto)	3s
tag-match (\d.\d, 0)	4s
tag-schedule	4s
tag-match (v(.*), 1)	5s
tag-schedule (cron-{{date 'YYYYMMDD'}})	3s
tag-semver (true)	6s
tag-schedule ({{date 'YYYYMMDD-HHmmss'}})	3s
tag-semver (false)	7s
tag-schedule (schedule)	4s
	2m 55s

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