message: add hack week notice
diff: # [Backstage](https://backstage.io)
  +_During March 7 to March 11 the maintainers will be taking part in Spotify's annual hack week. Development will continue as usual, but expect a slower pace for discussions and PR reviews. Why not take this opportunity to [build a plugin](https://backstage.io/docs/plugins/)?_
  +
  [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
  [![CNCF Status](https://img.shields.io/badge/cncf%20status-sandbox-blue.svg)](https://www.cncf.io/projects)
  [![Main CI Build](https://github.com/backstage/backstage/workflows/Main%20Master%20Build/badge.svg)](https://github.com/backstage/backstage/actions?query=workflow%3A%22Main+Master+Build%22)
  