added missing import of "io" in server example

Lutz Horn · emersion · commit 9ea8004c0fd1 · 2019-01-23T14:52:11.000+01:00
diff --git a/README.md b/README.md
@@ -57,6 +57,7 @@ package main
 
 import (
 	"errors"
+	"io"
 	"io/ioutil"
 	"log"
 
