doc: add README.md into cors/backend (#618)

cors/README.md

@@ -1,6 +1,6 @@
 # Cross-Origin Resource Sharing (CORS)
 
-## Run Server
+## [Run Server](backend/README.md)
 
 ```sh
 cd cors/backend

cors/backend/README.md

@@ -0,0 +1,26 @@
+# actix_cors::Cors
+
+## Running Server
+
+```sh
+cd cors/backend
+cargo run
+# starting HTTP server at http://localhost:8080
+```
+
+### web client
+
+- [http://localhost:8080/user/info](http://localhost:8080/user/info)
+  ```json5
+  // payload structure
+  {
+    "username": "username",
+    "email": "email",
+    "password": "password",
+    "confirm_password": "password"
+  }
+  ```
+
+## Others
+
+- For more related examples of [actix_cors](https://docs.rs/actix-cors/latest/actix_cors/struct.Cors.html)