SECURITY: add SECURITY.md

Change-Id: I37bce2704b1d5d466f6030f2b705149ed27248a7
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/235741
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
diff --git a/SECURITY.md b/SECURITY.md
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+# Security
+
+## Reporting a Vulnerability
+
+See https://golang.org/security for how to report a vulnerability.