Description in README,md probably should be updated

This entire paragraph talking about DXF, OFF and STL files seems incomplete and outdated. It has been written 16 years ago and OpenSCAD probably didn't support more formats back then. But now it does.

https://github.com/openscad/openscad/blob/12fe733b5a67d44b8b2de223dded6a31fc018ee3/README.md?plain=1#L28