Update README:

README.md

@@ -5,6 +5,9 @@
 
 This is a Captcha gem for Rails Applications which generates captcha image by C code.
 
+> NOTE: According to the use of Ruby China, the verification code looks like has a lower than 5% probability of being parsed by OCR and the verification code is cracked. It is recommended that you use the IP rate limit to enhance the protection.
+> NOTE: 以 Ruby China 的使用来看，验证码似乎有低于 5% 的概率被 OCR 读取解析导致验证码被破解（我们从日志分析绝大多数是成功的，但偶尔一个成功，配合大量机器攻击，导致注册了很多的垃圾账号），建议你额外配合 IP 频率限制的功能来加强保护。
+
 [中文介绍和使用说明](https://ruby-china.org/topics/27832)
 
 ## Example
@@ -47,7 +50,7 @@ RuCaptcha.configure do
   # self.length = 5
   # enable/disable Strikethrough.
   # self.strikethrough = true
-  # enable/disable Outline style, for hard mode
+  # enable/disable Outline style
   # self.outline = false
 end
 ```