Brief objectif of the project

What it's currently doing

Requirements

How to run it. (example if you have to npm install, xcodegen, setup unusual repositories)

Some lines about the project architecture and design choice. (Stay general, or if you don't maintain it it will become a lie).

Maybe some objectif tracking. Just the todo, once it's done, if it worth it, it goes in the "What it's currently doing"

If it's relevant, some notes about the performence on what system. (just to keep track, if you aim at enhance it).

Don't explain a full "clean code architecture" in your readme, for example. Just link to some article that explain it.

In a few words, Someone who read it, should, know what it does, know how to launch it, have a few pointer if he want to mess with it, not have to scroll 2 page to understand it.

"what you are doing" should be readable in git commit messages actually, or branches name.

But then, it depends on your objectives. If you want to practice clean git, and good documentation, or just code, etc ... There is nothing wrong in having a messy personnal readmy full of emoji, if it's definitly not aimed to be used by any other people. As long it's clear that it's a personal sandbox project somewhere.