📝 Add a GitHub-specific security policy note

PR #56866

It'll show up at
https://github.com/ansible/ansible/security/policy
and will help us guide users to the proper docs page

Co-authored-by: Alicia Cozine <879121+acozine@users.noreply.github.com>

.github/SECURITY.md

@@ -0,0 +1,15 @@
+# Security Policy
+
+## Supported Versions
+
+Ansible applies security fixes according to the 3-versions-back support
+policy. Please find more information in [our docs](
+https://docs.ansible.com/ansible/devel/reference_appendices/release_and_maintenance.html#release-status
+).
+
+## Reporting a Vulnerability
+
+We encourage responsible disclosure practices for security
+vulnerabilities. Please read our [policies for reporting bugs](
+https://docs.ansible.com/ansible/devel/community/reporting_bugs_and_features.html#reporting-a-bug
+) if you want to report a security issue that might affect Ansible.