Add instructions on how to report security issues

SECURITY.md

@@ -0,0 +1,8 @@
+# How to report a security issue with the gnome-sdk snaps
+
+The easiest way to report a security issue is through [GitHub](https://github.com/ubuntu/gnome-sdk/security/advisories/new).
+See [Privately reporting a security vulnerability](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing/privately-reporting-a-security-vulnerability)
+for instructions.
+
+The [Ubuntu Security disclosure and embargo policy](https://ubuntu.com/security/disclosure-policy)
+contains more information about what you can expect when you contact us and what we expect from you.