added a section in HTTP 1.0 outputs to README.

README.markdown

@@ -962,6 +962,42 @@ For instance,
     local res = ndk.set_var.set_escape_uri('a/b');
     -- now res == 'a%2fb'
 
+Trouble Shooting
+================
+
+HTTP 1.0 responses
+------------------
+
+Sometimes you may want to use nginx's ngx_proxy module to proxy requests to
+another nginx machine which configured a location with `content_by_lua`. Because
+`proxy_pass` only supports the HTTP 1.0 protocol, you have to pre-calculate
+the length of your response body and set the `Content-Length` header before emitting
+any data out. Here is a small example:
+
+On machine A:
+
+    location /internal {
+        rewrite ^/internal/(.*) /lua/$1 break;
+        proxy_pass http://B;
+    }
+
+then on machine B:
+
+    location = /lua/foo {
+        content_by_lua '
+            data = "hello, world"
+            ngx.header.content_length = #data
+            ngx.print(data)
+        ';
+    }
+
+Then accessing machine A's /internal/foo using curl gives the result that we expect.
+
+But some caveats apply here:
+
+1. Always set the `Content-Length` header *before* calling `ngx.print()` or `ngx.say()`.
+2. Only send out the response body data in a single call of `ngx.print()` or `ngx.say()`.
+
 Performance
 ===========