Add initial SECURITY.md (#203)

Co-authored-by: Tianon Gravi <[email protected]>

Author: greggles

SECURITY.md

@@ -0,0 +1,5 @@
+# Security Policy
+
+If you believe you have found a security issue with Drupal itself, please follow [the Drupal security issue process](https://www.drupal.org/docs/develop/issues/issue-procedures-and-etiquette/reporting-a-security-issue).
+
+If your issue is specific to the Dockerization / container image, [see the instructions in the umbrella "Official Images" `SECURITY.md` file](https://github.com/docker-library/official-images/tree/HEAD/SECURITY.md) for how to reach the @docker-library organization maintainers privately.