add a security policy (#3733)

* add a security policy

* use GitHub's private disclosure feature

SECURITY.md

@@ -0,0 +1,19 @@
+# Security Policy
+
+quic-go still in development. This means that there may be problems in our protocols,
+or there may be mistakes in our implementations.
+We take security vulnerabilities very seriously. If you discover a security issue,
+please bring it to our attention right away!
+
+## Reporting a Vulnerability
+
+If you find a vulnerability that may affect live deployments -- for example, by exposing
+a remote execution exploit -- please [**report privately**](https://github.com/quic-go/quic-go/security/advisories/new).
+Please **DO NOT file a public issue**.
+
+If the issue is an implementation weakness that cannot be immediately exploited or
+something not yet deployed, just discuss it openly.
+
+## Reporting a non security bug
+
+For non-security bugs, please simply file a GitHub [issue](https://github.com/quic-go/quic-go/issues/new).