Add special rules for document types

[wooorm]

• readme: idea: standard-readme standard/standard#141 has a great list of useful “readme rules”;
• history:
• maybe validate the headings to a certain standard (\d\.\d\.\d \/ \d{4}-\d{2}-\d{2});
• maybe require a commit reference at the end of a list-item?
• \.\d — man section document (e.g., mdast.1.md):
• Have a main heading in the form of \w+ \(\d\) -- .+
• Have a synopsis heading (maybe validate its content?);
• Have a description, bugs, and author heading;

...more?

[wooorm]

If people want this they should create external rules.