I find it useful to include an architecture diagram in the README for small projects, and the best way is to use the VSCode DrawIO extension. You can directly edit .drawio.svg files and embed them into the README. You get live editing and up-to-date images at the same time!
I've been recently using pic to draw any kind of images for documentation purposes.
Pic is a language Brian Kernighan created for troff toolchain. It's very versatile and has macros, even! Manual: https://pikchr.org/home/uv/pic.pdf
Contemporary implementations (pic2plot, dpic, pikchr) can output SVG so the results look very nice and are repository-friendly and diff-friendly to some extent.
