add some tip in rules about non-unicode response

rule_sample/rule__blank.js

@@ -98,7 +98,7 @@ module.exports = {
     //替换服务器响应的数据
     //replace the response from the server before it's sent to the user
     //you may return either a Buffer or a string
-    //serverResData is a Buffer, you may get its content by calling serverResData.toString()
+    //serverResData is a Buffer. for those non-unicode reponse , serverResData.toString() should not be your first choice.
     replaceServerResDataAsync: function(req,res,serverResData,callback){
         callback(serverResData);
     },

rule_sample/rule_replace_response_data.js

@@ -4,6 +4,9 @@ module.exports = {
 
     replaceServerResDataAsync: function(req,res,serverResData,callback){
         //append "hello world" to all web pages
+
+        //for those non-unicode response , serverResData.toString() should not be your first choice.
+        //this issue may help you understanding how to modify an non-unicode response: https://github.com/alibaba/anyproxy/issues/20
         if(/html/i.test(res.headers['content-type'])){
             var newDataStr = serverResData.toString();
             newDataStr += "hello world!";