Use the QuickTime (QTtxt) transcript format. Please take a look at this subtitle sample from Lynda.com.
How to Quickly Save Using the same Filename as the Corresponding Video
The following instructions apply to Windows users but may have equivalent steps in other operating systems.
- In the Save window, in the Save as Type dropdown selection, select All Files (*.*) and select the video file intended for the subtitle that you are saving. This will change the current filename in the filename field to the same filename as the video filename.
- In the Save as Type dropdown selection again, select the item with (*.srt). This will change the current file extension in the filename field to .srt.