doc: get request_id from header (#38)

README.md

@@ -90,3 +90,29 @@ r.GET("/", func(c *gin.Context) {
   c.String(http.StatusOK, "id:"+requestid.Get(c))
 })
 ```
+
+You can also get the request identifier from response header:
+
+```
+> curl -i "http://127.0.0.1:8080"
+
+HTTP/1.1 200 OK
+Content-Type: text/plain; charset=utf-8
+X-Request-ID: 77966910-3912-4193-9b74-267491c51700
+Content-Length: 39
+
+id:77966910-3912-4193-9b74-267491c51700
+```
+
+When http request with custom identifier, gin server return the custom identifier in response header.
+
+```
+> curl -i -H "X-Request-ID:test" "http://127.0.0.1:8080"
+
+HTTP/1.1 200 OK
+Content-Type: text/plain; charset=utf-8
+X-Request-Id: 1688221042
+Content-Length: 13
+
+id:test
+```