I've been meaning to write something like this for at least six years, to document how ICU integrates into our build(s), how to correctly update it, (more recently) how to update tzdata, and how to regenerate locale data.
And last Thursday, for personal reasons I'm happy to explain in person but will not explain here, I needed to write some documentation -- any documentation. So I wrote some about ICU that has long needed doing.
It took a few days, but I've finished that documentation. It should be helpful into the future so ICU and related updates don't require me going off my vague memories, random Bugzilla comments, or anba's own knowledge of how to do it. Let's do this.
I don't really know where we want to keep documentation these days, or in what format, or viewable using what viewer. I wrote stuff up in Markdown because it seemed...plausibly a sensible format? And lots of imported code has README.md sorts of files in it already? Anyway. I don't care what the format is long-term, but right now I kind of want to get something landed, and we can change format after if there's reason. (Like, perhaps, more ability to not store text with massively long lines, so it's more editable and reviewable. So it goes, for now.)