Update security.md to follow GitHub template (#2325)

* Update security.md to follow GitHub template

* Update wording in SECURITY.md for clarity

Author: mathbunnyru

SECURITY.md

@@ -1,4 +1,9 @@
-# Reporting a Vulnerability
+# Security Policy
 
-If you believe you’ve found a security vulnerability in a Jupyter project, please report it!
-See the [security documentation](https://jupyterhub.readthedocs.io/en/latest/contributing/security.html) for how.
+## Supported Versions
+
+Jupyter Docker Stacks only provides security updates for the latest version of each image.
+
+## Reporting a Vulnerability
+
+The Jupyter Vulnerability Handling Process is described in detail in the [security documentation](https://github.com/jupyter/security/blob/main/docs/vulnerability-handling.md).