Add security instructions

Author: kylekatarnls

README.md

@@ -26,3 +26,11 @@ $html = $renderer->render($pugInput);
 
 //$html is now a string of HTML or any other markup according to the formatter you choose (XML, xHTML, etc.)
 ```
+
+Security contact information
+-----
+
+To report a security vulnerability, please use the
+[Tidelift security contact](https://tidelift.com/security).
+Tidelift will coordinate the fix and disclosure.
+