docs: add security policy

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only the currently deployed site, built from `master` and published to
+https://security.libredevops.org, receives security updates.
+
+Tags in this repository are historical markers rather than supported releases.
+There is no backport process: fixes land on `master` and reach the live site on
+the next deployment.
+
+## Scope
+
+Security News is a static site with no backend, no accounts and no server-side
+request handling. Feeds are ingested by a scheduled GitHub Actions run and the
+result is published to GitHub Pages.
+
+In scope:
+
+- The feed ingestion script (`scripts/fetch_feeds.py`) and its dependencies
+  (`scripts/requirements.txt`).
+- The front end: `index.html`, `js/`, `css/`, and the service worker (`sw.js`).
+- The generated artefacts in `data/` (`feeds.json`, `feed.xml`), including
+  injection or escaping issues arising from untrusted upstream feed content.
+- The workflows in `.github/workflows/`, including their permissions and any
+  path allowing an untrusted input to influence a commit or deployment.
+- The deployed site and feed at https://security.libredevops.org.
+
+Out of scope:
+
+- The content of aggregated articles and the upstream vendor feeds themselves.
+  Report those to the originating vendor — this project only republishes them.
+- Inaccurate, missing, stale or miscategorised articles. These are correctness
+  bugs, not vulnerabilities; raise a normal issue.
+- Vulnerabilities in third-party dependencies that already carry a public
+  advisory. Dependabot tracks those automatically, so a report adds nothing.
+
+Because this project aggregates security content, a plausible finding is one
+where hostile upstream feed content reaches a viewer's browser or influences the
+build — stored XSS through an article title or summary, HTML or XML injection
+into `feed.xml`, service worker cache poisoning, or a workflow that can be
+steered by feed data.
+
+## Reporting a Vulnerability
+
+Report privately using GitHub's private vulnerability reporting:
+
+**https://github.com/libre-devops/security-news/security/advisories/new**
+
+Do **not** open a public issue for an undisclosed vulnerability.
+
+Please include:
+
+- The affected component and, where relevant, the commit or deployment date.
+- Reproduction steps, ideally with the specific feed entry or payload involved.
+- The impact you believe it has.
+- Any suggested remediation, if you have one.
+
+## What to Expect
+
+- Acknowledgement of receipt within **3 business days**.
+- An initial triage decision within **7 business days**.
+
+If the report is accepted, we will develop a fix and coordinate disclosure
+timing with you once a patch or mitigation is deployed. If it is declined, we
+will tell you why — for example not reproducible, out of scope as described
+above, or already publicly known.
+
+Please hold off on public disclosure until remediation is complete. This is a
+volunteer-maintained project, so please be reasonable about timelines.
+
+Published advisories:
+**https://github.com/libre-devops/security-news/security/advisories**