Apparently you cannot turn off the annoying shutter sound,there are no options.But with the new Android 6.0.1 update they have provided an option of switching off the shutter sound which will be visible once you scroll through the flash,HDR and other options when the camera app is working. There you can see an option of deselcting shutter sound.
A simple workaround is to lower the ringer volume. The camera shutter sound volume follows ringer’s volume.
You can press the volume down button when there are no apps are opened. This will adjust ringer volume.
If camera shutter sound does not follow the settings, you may try to clear cache and clear data for camera app in Settings– Apps–Camera.