Create a skeleton VISUALS.md file for GIFs, etc...
Create a skeleton VISUALS.md file for GIFs, etc...

Reasoning: It feels sensible to have a dedicated file for GIFs (and
perhaps other media content pending future contributions) instead of
flooding the main README with images and videos.
This is currently a very bare-bones file, listing all the GIFs I have
available in the order the corresponding VimTeX features appear in
`:help vimtex-features`. Since the order in both files matches up, I'm
currently imagining a new user might simultaneously scroll through this
file while reading `:help vimtex-features` and thus get a better image
than either file along could provide.

Note: I see that `github.com/lervag/vimtex-media` already exists. I have
temporarily created my own `github.com/ejmastnak/vimtex-media`, since I
see the content of `github.com/lervag/vimtex-media` is MIT-licensed, and
I had already released the GIFs I made under a CC-BY-NC-SA (sigh).
I imagine we can work this out easily (I'm not one for splitting hairs
over licenses), but I did want to wait for input before making a
committal decision of my own.

I do feel that a CC license would be more appropriate for media (MIT is
for code, no?), but that might(?) require `@DustyTopology`'s consent,
and I think his GitHub account is deleted. Eh, we'll figure something
out.

lervag/vimtex,lervag/vimtex