Add instructions for custom fonts to readme

[nucleic-acid]

Hi, thanks for providing this extension/template. I never managed to implement this on my own and just recently found your extension.

I figured out, how to use custom fonts in the letter (I personally don't like lmodern too much).
Can I open a PR to add the details to your readme?

The font files need to be in the root folder of the *.qmd file.
Supposing the font files are called e.g. "roboto-latin-regular.ttf" or "roboto-latin-italic.ttf", etc.,
the necessary YAML options are:

mainfont: roboto
mainfontoptions:
- Extension=.ttf
- UprightFont=*-latin-regular
- BoldFont=*-latin-700
- ItalicFont=*-latin-italic
- BoldItalicFont=*-latin-700italic

This changes the font for the body, header, the address..., basically all except the back-address-field.
To change the latter, you need to also add the following:

sansfont: roboto
sansfontoptions:
- Extension=.ttf
- UprightFont=*-latin-regular
- BoldFont=*-latin-700
- ItalicFont=*-latin-italic
- BoldItalicFont=*-latin-700italic

[mavam]

Thanks for reaching out! I had something like this on my todo list, but never got to it. A PR would be very much welcome! 🙏

I wonder whether we should move the fonts and other assets into a separate directory. Having one "theme" that hides the details would be the ultimate UX, but that would another layer of indirection.

[nucleic-acid]

Moving the fonts would be the cleaner option, but I couldn't get it to work. So I'll open a PR soon and maybe we can work out a better solution at a later point?

[mavam]

Yeah, instructions on how to get started on fonts are still a good start. Thanks again for the contribution!