Add Breaking Change / Changelog / Migration Guide explanation in the Contributor's Guide

The Changelog/Migration Guide sections are pre-filled in the placeholder when creating a PR, but they're not mentioned anywhere in the [Contributor's Guide](https://github.com/bevyengine/bevy/blob/main/CONTRIBUTING.md).

They should be mentioned as they are used by scripts in the release process, more and more without a human review since the number of PRs per version is growing.

Guidelines about how to write a Migration Guide and/or changelogs would help normalize them and allow us to rely more on scripts too.