[enh] add security policy

SECURITY.md

@@ -0,0 +1,10 @@
+# Security Policy
+
+We love responsible reports of (potential) security issues in SearXNG.
+
+You can contact us at security@searxng.org.
+
+Be sure to provide as much information as possible and if found
+also reproduction steps of the identified vulnerability. Also
+add the specific URL of the project as well as code you found
+the issue in to your report.