Updated readme file

README.md

@@ -62,6 +62,15 @@ with s3_fs.open('s3://csst-prod/gaia/test/requirements.txt', s3_options=s3_optio
     file.write("CSST")
 ```
 
+### Check if the given file path exists
+```python
+from csst_fs import fs
+# local or on s3, depending on the given path
+fs.isfile('requirements.txt')
+fs.isfile('s3://csst-prod/test.txt')
+fs.isfile('s3://csst-prod/test.txt', s3_options=s3_options)
+```
+
 ## astropy直接读写s3的写法适配
 ### fits.open
 #### 老写法