Apply suggestion for security policy, copied from Pylons

See https://github.com/Pylons/.github/blob/main/SECURITY.md
See https://github.com/collective/icalendar/pull/755#discussion_r1894708114

docs/security.rst

@@ -15,20 +15,42 @@ Security vulnerabilities are fixed only for the latest version of ``icalendar``.
    * - Version
      - Supported
    * - 6.*
-     - ✅
+     - YES
    * - 5.*
-     - ❌
+     - no
    * - 4.*
-     - ❌
+     - no
    * - < 4.*
-     - ❌
+     - no
 
 
 Reporting a Vulnerability
 -------------------------
 
-Please `report vulnerabilities of icalendar to Plone
-<https://github.com/plone/.github/blob/main/SECURITY.md#readme>`_.
-If you cannot do this, please contact one of the
-:ref:`maintainers`
-directly or open an issue.
+To report security issues of ``collective/icalendar``, use the ``Report a vulnerability`` button on the project's `Security Page <https://github.com/collective/icalendar/security>`_.
+If you cannot do this, please contact one of the :ref:`maintainers` directly.
+
+If we determine that your report may be a security issue with the project, we may contact you for further information.
+We volunteers ask that you delay public disclosure of your report for at least ninety (90) days from the date you report it to us.
+This will allow sufficient time for us to process your report and coordinate disclosure with you.
+
+Once verified and fixed, the following steps will be taken:
+
+-   We will use GitHub's Security Advisory tool to report the issue.
+-   GitHub will review our Security Advisory report for compliance with Common Vulnerabilities and Exposures (CVE) rules.
+    If it is compliant, they will submit it to the MITRE Corporation to generate a `CVE <https://www.cve.org/>`_.
+    This in turn submits the CVE to the `National Vulnerability Database (NVD) <https://nvd.nist.gov/vuln/search>`_.
+    GitHub notifies us of their decision.
+-   Assuming it is compliant, we then publish our Security Advisory on GitHub, which triggers the next steps.
+-   GitHub will publish the CVE to the CVE List.
+-   GitHub will broadcast our Security Advisory via the `GitHub Advisory Database <https://github.com/advisories`_.
+-   GitHub will send `security alerts <https://docs.github.com/en/code-security/supply-chain-security/managing-vulnerabilities-in-your-projects-dependencies/about-alerts-for-vulnerable-dependencies>`_ to all repositories that use our package (and have opted into security alerts).
+    This includes Dependabot alerts.
+-   We will make a bug-fix release.
+-   We will send an announcement through our usual channels:
+
+    - The GitHub release
+    - The GitHub discussions
+    - The `Plone Community Forum <https://community.plone.org/>`_
+
+-   We will provide credit to the reporter or researcher in the vulnerability notice.