Documentation: Exaustive list of all acceptable image file formats

[VictorGaiva]

In my applications, it makes sense to verify if the file extension is acceptable by Vix before calling Image.new_from_file, so that I would not have to download a file I already know won't be processable.

I was unable to find this information on the documentation, neither on the code.

[akash-akya]

Hey @VictorGaiva, precompiled libvips which comes with vix by default supports -- svg, png, tiff, jpeg, webp, raw, heif, gif image formats.

I'll update the documentation.

[akash-akya]

Updated documentation in #134
Thanks!