Update README.md

Author: chyyuu

README.md

@@ -5,4 +5,5 @@
 - [OS API docs of ch4](https://learningos.github.io/rCore-Tutorial-Code-2022S/ch4/os/index.html)
 - [OS API docs of ch5](https://learningos.github.io/rCore-Tutorial-Code-2022S/ch5/os/index.html)
 - [OS API docs of ch6](https://learningos.github.io/rCore-Tutorial-Code-2022S/ch6/os/index.html)
+   - [easyFS API doc in ch6]( https://learningos.github.io/rCore-Tutorial-Code-2022S/ch6/easy_fs/index.html) 
 - [OS API docs of ch7](https://learningos.github.io/rCore-Tutorial-Code-2022S/ch7/os/index.html)