Document usage of Named Regex groups in replacement patterns

[AndreyNautilus]

It would be great to document the usage of Named Regex groups in "replacementPatterns".
For example:

{
  "replacementPatterns": [
    {
      "pattern": "assets/(?<filename>.*)",
      "replacement": "images/$<filename>"
    }
  ]
}

will replace assets/hello.jpg with images/hello.jpg before validation. It's a standard behavior of JS string.replace method, but for users who don't know that the checker is written in JS and that it uses string.replace, this is a hidden knowledge (I've spent quite some time reading the code and the documentation). Better to document that explicitly (with relevant links) to save the time for future users.

I can create a PR to show the change.

And thanks for a great tool, BTW!