feat: $comment keyword

[andriskaaz]

Please:

• [x] Make your pull request atomic, fixing one issue at a time unless there are many relevant issues that cannot be decoupled.
• [x] Provide a test case & update the documentation in the Readme.md

JSON Schema Docs: https://json-schema.org/understanding-json-schema/reference/comments

[domoritz]

Can you add a link to the docs to the pull request description?

[andriskaaz]

@domoritz I've updated the description.