Add vendor warning

Author: bradfitz

tcpproxy.go

@@ -37,6 +37,9 @@
 // Note that tcpproxy does not do any TLS encryption or decryption. It
 // only (via DialProxy) copies bytes around. The SNI hostname in the TLS
 // header is unencrypted, for better or worse.
+//
+// This package makes no API stability promises. If you depend on it,
+// vendor it.
 package tcpproxy
 
 import (