message: Document bugfix for the I/O write backoff handling
diff: All notable changes to this project will be documented in this file. The format
  is based on [Keep a Changelog](https://keepachangelog.com).
  +## Unreleased
  +
  +### Fixed
  +
  +- 0.18.4 introduced a potential crash when using the OpenSSL module and
  + encountering `SSL_ERROR_WANT_READ`. The crash manifested if CAF resumed a
  + write operation but failed to fully reset its state. The state management (and
  + consequently the crash) has been fixed.
  +
  ## [0.18.4] - 2021-07-07
  ### Added
  