docs: add security policy

docs/security.md

@@ -0,0 +1,7 @@
+To privately report a security issue with our projects, please send an email to:
+
+[admin@nix-community.org](mailto:admin@nix-community.org)
+
+We will usually respond within 48 hours.
+
+Thank you.

mkdocs.yml

@@ -7,6 +7,7 @@ nav:
       - FAQ: faq.md
       - administrators.md
   - contact.md
+  - security.md
 
 theme:
   name: material