Add ability to include specific files in searchPaths

[kevbook]

Certain repos don't have an actual docs, just readme.md
See example:

    {
      "type": "git",
      "name": "ky",
      "url": "https://github.com/sindresorhus/ky",
      "branch": "main",
      "searchPaths": ["readme.md", "source"]
    },