header structure for rules in the README

[acao]

the headers structure for the rules seems irregular and it made it trickier to read, so i set them all to the initial headers structure used for the firs two rules. would you agree that it's harder to see the separation between rules when using h3/h4?

Other ideas to improve readability:

• blockquote the rule description to make it stand out?
• using the collapsing markdown sections GFM supports? doesn't work on npm though?

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