README: mention supported PHP versions #59

Job	Run time
checks	25s
tests (7.2, prefer-lowest, ubuntu-latest)	13s
tests (7.2, prefer-lowest, windows-latest)	35s
tests (7.2, prefer-stable, ubuntu-latest)	10s
tests (7.2, prefer-stable, windows-latest)	37s
tests (7.3, prefer-lowest, ubuntu-latest)	16s
tests (7.3, prefer-lowest, windows-latest)	40s
tests (7.3, prefer-stable, ubuntu-latest)	15s
tests (7.3, prefer-stable, windows-latest)	38s
tests (7.4, prefer-lowest, ubuntu-latest)	14s
tests (7.4, prefer-lowest, windows-latest)	36s
tests (7.4, prefer-stable, ubuntu-latest)	10s
tests (7.4, prefer-stable, windows-latest)	34s
tests (8.0, prefer-lowest, ubuntu-latest)	14s
tests (8.0, prefer-lowest, windows-latest)	31s
tests (8.0, prefer-stable, ubuntu-latest)	14s
tests (8.0, prefer-stable, windows-latest)	41s
tests (8.1, prefer-lowest, ubuntu-latest)	12s
tests (8.1, prefer-lowest, windows-latest)	36s
tests (8.1, prefer-stable, ubuntu-latest)	19s
tests (8.1, prefer-stable, windows-latest)	37s
tests (8.2, prefer-lowest, ubuntu-latest)	11s
tests (8.2, prefer-lowest, windows-latest)	35s
tests (8.2, prefer-stable, ubuntu-latest)	11s
tests (8.2, prefer-stable, windows-latest)	31s
tests (8.3, prefer-lowest, ubuntu-latest)	9s
tests (8.3, prefer-lowest, windows-latest)	37s
tests (8.3, prefer-stable, ubuntu-latest)	15s
tests (8.3, prefer-stable, windows-latest)	41s
	11m 57s

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