Serve easily dynamic files with DataFromReader context method (#1304)

* Add DataFromReader context method

* Replace fmt by strconv.FormatInt

* Add pull request link to README

README.md

@@ -40,6 +40,7 @@ Gin is a web framework written in Go (Golang). It features a martini-like API wi
     - [XML, JSON and YAML rendering](#xml-json-and-yaml-rendering)
     - [JSONP rendering](#jsonp)
     - [Serving static files](#serving-static-files)
+    - [Serving data from reader](#serving-data-from-reader)
     - [HTML rendering](#html-rendering)
     - [Multitemplate](#multitemplate)
     - [Redirects](#redirects)
@@ -901,6 +902,32 @@ func main() {
 }
 ```
 
+### Serving data from reader
+
+```go
+func main() {
+	router := gin.Default()
+	router.GET("/someDataFromReader", func(c *gin.Context) {
+		response, err := http.Get("https://raw.githubusercontent.com/gin-gonic/logo/master/color.png")
+		if err != nil || response.StatusCode != http.StatusOK {
+			c.Status(http.StatusServiceUnavailable)
+			return
+		}
+
+		reader := response.Body
+		contentLength := response.ContentLength
+		contentType := response.Header.Get("Content-Type")
+
+		extraHeaders := map[string]string{
+			"Content-Disposition": `attachment; filename="gopher.png"`,
+		}
+
+		c.DataFromReader(http.StatusOK, contentLength, contentType, reader, extraHeaders)
+	})
+	router.Run(":8080")
+}
+```
+
 ### HTML rendering
 
 Using LoadHTMLGlob() or LoadHTMLFiles()