Document non JPG/PNG format handling

[Sander van Dragt]

Couldn't find in the documentation. Noticed in the code that GIF is returned as is.

Perhaps a good place for the docs is https://docs.altis-dxp.com/media/dynamic-images/, maybe also document it on this repo.

[Joe Hoyle]

This is fixed in #162 as we now support GIF image resizing.