If you maintain an open-source project in the range of 10k-200k lines of code, I strongly encourage you to add an ARCHITECTURE document next to README and CONTRIBUTING.
Before going into the details of why and how, I want to emphasize that this is not another docs are good, write more docs advice.
I am pretty sloppy about documentation, and, e.g., I often use just simplify as a commit message.
Nonetheless, I feel strongly about the issue, even to the point of pestering you :-)

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Adding an ARCHITECTURE document to open-source projects can bridge the gap between contributors and core developers by providing a high-level overview of the project's structure. The document should include a bird's eye overview, a codemap of coarse-grained modules, and explicit architectural invariants. It should be revisited a few times a year to ensure accuracy.

ARCHITECTURE.md