[action-publish-release] Document `release.config.json`

When operating on a monorepo, this action looks for {{release.config.json}} in the root to determine the desired versioning strategy and act accordingly.

* For an “independent” strategy (all packages are allowed to have their own versions), the action will create tags for each package released, and the release notes will include the version for each package released.
* For a “fixed” strategy (all package versions are kept in sync for each release), the action won’t create tags for each package release — it will just create one tag to represent the synced version — and the release notes will not include the version for each package released.

The README should mention what {{release.config.json}} is, what the versioning strategy represents and how that affects this action, and what the default strategy is for a monorepo.

Start by reading the README and searching it for existing references to release.config.json. Document what the file is, explain the independent and fixed strategies and their effect on tags and release notes, and state the default monorepo strategy. Done means a newcomer can understand the configuration and its release behavior from the README.

Written by the indexing model from the issue text.