message: Document fix for regression
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]
  +## [0.18.1] - 2021-03-19
  +
  +### Fixed
  +
  +- Version 0.18.0 introduced a regression on the system parameter
  + `caf.middleman.heartbeat-interval` (#1235). We have addressed the issue by
  + porting the original fix for CAF 0.17.5 (#1095) to the 0.18 series.
  ## [0.18.0] - 2021-01-25
  