doc: Add Sentinel website in README.md (#2054)

README.md

@@ -28,6 +28,8 @@ Features overview:
 
 ## Documentation
 
+See the [Sentinel](https://sentinelguard.io/) for the document website.
+
 See the [中文文档](https://github.com/alibaba/Sentinel/wiki/%E4%BB%8B%E7%BB%8D) for document in Chinese.
 
 See the [Wiki](https://github.com/alibaba/Sentinel/wiki) for full documentation, examples, blog posts, operational details and other information.