README: use the xmllint-validate action in the CI code sample#174

jrfnl merged 1 commit intodevelopPHPCSStandards/PHPCSDevTools:developfrom

feature/readme-update-docs-xsd-ci-code-samplePHPCSStandards/PHPCSDevTools:feature/readme-update-docs-xsd-ci-code-sampleCopy head branch name to clipboard