README: add a notice about whether this action is needed #338

Job	Run time
selftest (ubuntu-latest)	20s
selftest (macos-latest)	20s
selftest-runner-python (ubuntu-latest)	25s
selftest (windows-latest)	1m 0s
selftest-whitespace (ubuntu-latest)	20s
selftest-runner-python (ubuntu-24.04)	19s
selftest (ubuntu-24.04)	24s
selftest-xfail-invalid-inputs (--this-should-not-work)	20s
selftest-whitespace (macos-latest)	24s
selftest-xfail-invalid-inputs (/tmp/extremely-nonexistent-file)	20s
selftest-whitespace (windows-latest)	50s
selftest-glob-multiple	22s
selftest-glob	20s
selftest-staging	23s
selftest-xfail-glob-input-expansion	20s
selftest-identity-token	22s
selftest-upload-artifacts	19s
selftest-xfail-verify-missing-options (true)	19s
selftest-verify	21s
selftest-xfail-verify-missing-options (true, https://token.actions.githubusercontent.com)	19s
selftest-xfail-verify-missing-options (true, https://github.com/sigstore/gh-action-sigstore-pytho...	18s
selftest-xfail-verify-missing-options (false, https://token.actions.githubusercontent.com)	19s
selftest-xfail-verify-missing-options (false, https://github.com/sigstore/gh-action-sigstore-pyth...	17s
all-selftests-pass	1s
	9m 2s

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