Update query-metadata-style-guide.md
Update query-metadata-style-guide.md

Add a note about the `@security-severity` tag.
github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql