docs: add security policy for private vulnerability reports

.github/ISSUE_TEMPLATE/config.yml

@@ -3,3 +3,6 @@ contact_links:
   - name: Download the latest release
     url: https://github.com/l0ng-ai/openidea/releases/latest
     about: Many problems are already fixed. Please check the latest version before reporting.
+  - name: Report a security vulnerability
+    url: https://github.com/l0ng-ai/openidea/security/advisories/new
+    about: Please report security problems privately, not in a public issue.

SECURITY.md

@@ -0,0 +1,26 @@
+# Security policy
+
+## Reporting a vulnerability
+
+Please do not report security problems in public issues, pull requests or discussions.
+
+Report them privately through GitHub:
+[Report a vulnerability](https://github.com/l0ng-ai/openidea/security/advisories/new)
+(repository **Security** tab → **Report a vulnerability**). Only the maintainers can see
+the report.
+
+Include what you can of:
+
+- the affected version, platform, and whether remote development (`openidea-server`)
+  or plugins are involved;
+- steps to reproduce, or a proof of concept;
+- the impact you expect, and any fix you suggest.
+
+We will acknowledge the report, keep you updated in the advisory, and agree on a
+disclosure date with you. Fixes are released as a new version, and the advisory is
+published once users can update.
+
+## Supported versions
+
+Security fixes go into the latest release. Please check that the problem still occurs
+there before reporting.