Update README.md

README.md

@@ -109,6 +109,12 @@ _, err = mp4.ReadBoxStructure(r, func(h *mp4.ReadHandle) (interface{}, error) {
 })
 ```
 
+Please note that the above sample code doesn't work for some MP4 files.
+If your MP4 files includes specific box types (ex. stco, mfra), you should update those when offsets of mdat box changed.
+Next sample code adds an metadata box and updates chunk offsets in scto boxes.
+
+[Sample Code: Insert Metadata and Update stco box](https://gist.github.com/sunfish-shogi/cccde016a38c66d32c07a0234368804e)
+
 ### User-defined Boxes
 
 You can create additional box definition as follows: