Add API reference

README.md

@@ -73,6 +73,7 @@ A lite http request lib based on **fetch** with plugin support and similar API t
   - [Sending a request with credentials included](#sending-a-request-with-credentials-included)
   - [Uploading a file](#uploading-a-file)
   - [Processing a text file line by line](#processing-a-text-file-line-by-line)
+- [API Reference](#api-reference)
 - [Star History](#star-history)
 - [Thanks](#thanks)
 
@@ -2187,6 +2188,10 @@ async function run() {
 run();
 ```
 
+## API Reference
+
+https://suhaotian.github.io/xior/
+
 ## Star History
 
 [![Star History Chart](https://api.star-history.com/svg?repos=suhaotian/xior&type=Date)](https://star-history.com/#suhaotian/xior&Date)