Add documentation for detectable breaking/non-breaking changes

We are using this library to power a GitHub Action that generates a backward compatibility report between two versions of an OpenAPI spec. We love this library, but it is not clear what changes are detectable by this library, and what changes this library classifies as "breaking" or "non-breaking" changes. It would be really valuable to us if this repo could include a markdown file outlining all the OAS changes that this library can detect.

未指定文件或测试。首先检查该库的 OpenAPI 比较行为，并盘点它能够检测到哪些规范变更，以及它如何将这些变更分类为 breaking 或 non-breaking。添加一份清晰概述可检测的 OAS 变更及其分类的 Markdown 文档后，即视为完成。