Add unix domain socket upstream

S3-Nginx-Proxy.md

@@ -5,6 +5,9 @@ It's a common concept to put a proxy in front of S3 that handles requests. Nginx
 ```
 upstream seaweedfs { server localhost:8333 fail_timeout=0; keepalive 20;}
 
+## Also you can use unix domain socket instead for better performance:
+# upstream seaweedfs { server unix:/tmp/seaweedfs-s3-8333.sock; keepalive 20;}
+
 server {
 	listen 443 ssl;
 	server_name ~^(?<subdomain>[^.]+).yourdomain.com;