docs: add guidance on unsupported architectures to README.md #288

Job	Run time
build (20.x, macos-latest)	50s
build (18.x, windows-latest)	2m 16s
build (18.x, macos-latest)	49s
test_host / test (18.x, windows-latest)	2m 49s
build (18.x, ubuntu-latest)	42s
build (20.x, ubuntu-latest)	31s
test_host / test (20.x, ubuntu-latest)	5m 57s
build (20.x, windows-latest)	1m 48s
test_host / test (20.x, macos-latest)	1m 43s
test_host / test (18.x, ubuntu-latest)	5m 52s
test_host / test (18.x, macos-latest)	1m 24s
test_host / test (20.x, windows-latest)	2m 19s
test_18 / test (18.x, ubuntu-latest)	3m 23s
test_18 / test (20.x, macos-latest)	3m 6s
test_20 / test (20.x, windows-latest)	7m 14s
test_20 / test (18.x, windows-latest)	5m 2s
test_20 / test (20.x, ubuntu-latest)	3m 4s
test_20 / test (18.x, ubuntu-latest)	3m 15s
test_18 / test (18.x, macos-latest)	3m 29s
test_20 / test (20.x, macos-latest)	2m 47s
test_20 / test (18.x, macos-latest)	3m 42s
test_18 / test (20.x, windows-latest)	1m 1s
test_18 / test (20.x, ubuntu-latest)	3m 4s
test_18 / test (18.x, windows-latest)	4m 48s
	1h 10m 55s

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