SECURITY.md: use private vulnerability reporting feature (#772)

SECURITY.md

@@ -1,5 +1,17 @@
 # Security Policy
 
+## Supported Versions
+
+Security updates are applied only to the most recent release.
+
 ## Reporting a Vulnerability
 
-Please report security issues to `bascule@gmail.com`
+If you have discovered a security vulnerability in this project, please report
+it privately. **Do not disclose it as a public issue.** This gives us time to
+work with you to fix the issue before public exposure, reducing the chance that
+the exploit will be used before a patch is released.
+
+Please disclose it at [security advisory](https://github.com/httprb/http/security/advisories/new).
+
+This project is maintained by a team of volunteers on a reasonable-effort basis.
+As such, please give us at least 90 days to work on a fix before public exposure.