Add metapackage names in README.md

[ld991]

Fix issues #68 and #69. Mention metapackages and add links to their official websites in a new table column in README.md. Three metapackages are listed, namely, FontForge for fontimage, ImageMagick for convert and Transmission for transmission-show.

This helps users easily discover and install programs required for file previews for various filetypes.

[NikitaIvanovV]

Thank you!

If we are going to do this, it would make sense to add links for more programs (glow, libreoffice, chafa, etc...).

I do not think a new table column is needed, we could add links for these programs in the second table column. Some programs are listed is multiple places, so I recommend using reference-style links like this:

[libreoffice][libreoffice]
[libreoffice][libreoffice]

[libreoffice]: https://www.libreoffice.org/

libreoffice
libreoffice

[ld991]

Yes, that does make more sense. And I also noticed the table appear in various places.

The issue I'm intending to fix is programs like fontimage and convert rather than programs like librewolf because for people who aren't already aware of the programs, even a Google search is unhelpful.

However, for the sake of consistency, adding a link to other programs also seems reasonable.

[ld991]

I removed the third table column and added links to almost all programs. The links were largely copied from Arch Linux package descriptions and include links to their websites or github projects.

[NikitaIvanovV]

And I also noticed the table appear in various places.

I completely forgot about that it also appears in the man page. And it's also auto-generated from previewer files!

I will make a change to the scripts that produce the tables.