Added Quad9 servers in config. Good for malware threat prevention.

doh-client/doh-client.conf

@@ -40,6 +40,11 @@ upstream_selector = "random"
 #    url = "https://1.1.1.1/dns-query"
 #    weight = 50
 
+## Quad9's resolver. Good for malware threat prevention.
+#[[upstream.upstream_ietf]]
+#    url = "https://9.9.9.9/dns-query"
+#    weight = 50
+
 ## Google's experimental resolver, good ECS, good DNSSEC
 #[[upstream.upstream_ietf]]
 #    url = "https://dns.google.com/experimental"