Document code structure and implementation

[waldyrious]

This is similar to #4, but instead of describing just the goals and motivation of the project (the "what" and "why"), this issue focuses on the "how".

Specifically, it would be nice to have an overview of that the code does and how it works. This documentation could live in README.md or in CONTRIBUTING.md (or both). I would expect at least a link to https://github.com/bobheadxi/readable/blob/main/plugins/readable/reflow.ts, which, if I understand correctly, is core of the project.