FileFromFS (#2112)

* Context.FileFromFS added

* Context File and FileFromFS examples at README

Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>

README.md

@@ -1182,6 +1182,24 @@ func main() {
 }
 ```
 
+### Serving data from file
+
+```go
+func main() {
+	router := gin.Default()
+
+	router.GET("/local/file", func(c *gin.Context) {
+		c.File("local/file.go")
+	})
+
+	var fs http.FileSystem = // ...
+	router.GET("/fs/file", func(c *gin.Context) {
+		c.FileFromFS("fs/file.go", fs)
+	})
+}
+
+```
+
 ### Serving data from reader
 
 ```go