message: Document modules removal in changelog
diff: +## Unreleased
  +
  +**Breaking changes**:
  +- ([#325](https://github.com/ramsayleung/rspotify/pull/325)) The `auth_code`, `auth_code_pkce`, `client_creds`, `clients::base` and `clients::oauth` modules have been removed from the public API; you should access the same types from their parent modules instead
  +
  ## 0.11.5 (2022.03.28)
  **Breaking changes**:
  