spec.html page is unclear on nb_frames

[ZoeyR]

In part 1 of the spec.html page the format of the first header is outlined. The header format specifies that there exists an nb_frames field that is 1 for still images or >2 for animated images. However the reference implementation produces FLIFs without that field present and assumes they don't exist on still images during decoding. I was able to discover this by performing a hexdump on a FLIF file output by the refernce flif converter. The documentation on the website should make it clear if this is the intended behavior or not.