[spec] adding a "magic number" for gemini files

[Sean Conner]

It was thus said that the Great cage once stated:
> On Sat, Dec 26, 2020 at 04:34:25PM +0100, Solderpunk wrote:
> > On Sat Dec 26, 2020 at 4:32 PM CET, cage wrote:
> >
> > > I think a lot of guesswork is needed (line finding link lines with
> > > gemini scheme?), for example, to discriminate between markdown and
> > > gemini file format.
> >
> > When is this really necessary?  Both Gemini and HTTP provide a means to
> > inform the client what's coming down the pipe.  For files distributed
> > otherwise, filename conventions would seem to do the trick.
> 
> I was  thinking abot file(1) and  libmagic in general.

  I use libmagic for my server [1] and yet, I still use an extension to mark
a Gemini text file.

  -spc

[1]	https://github.com/spc476/GLV-1.12556