Externally-visible metadata?

I've been working on an alternate implementation to be compliant with the AppImage type 2 spec using Zip as the image file system (https://github.com/sagebind/appimagezip), but now I realize there is no way for external tools to know what file system an AppImage actually uses. Would it be a good idea to add some metadata to a new ELF section?

I'm thinking something like this:

```
$ readelf -p .appimage MyApp.AppImage
name = My App
builder = AppImageKit
filesystem = squashfs
```

Being able to identify the file system type could be useful, so that tools don't have to guess the format by scanning the file. For example, if we want to extract an AppImage or list its contents, we would need to know the file system format first. It may also be useful to record the application used to generate the AppImage.

We can't store this metadata in the `.desktop` file since we can't access it without reading the file system format first.