Contributors shouldn't edit the CHANGELOG.md directly

[dbeatty10]

Describe the documentation change

We should clarify within our contributing guide that users should not make modifications to CHANGELOG.md anymore, because they will be overwritten during the release process.

While dbt-utils does not use changie like dbt-core, et al. it also doesn't need manual updating by users anymore.

The implementation can use dbt-core or dbt-snowflake for inspiration.

Additional context

#902 covered the PR checklist portion of this effort.

Who will this benefit?

This will benefit contributors and maintainers -- less to do all-around.