message: Promote Matrix room in README
diff: [pikelet-mascot]: assets/pikelet.png
  [![Actions Status][actions-badge]][actions-url]
  -[![Gitter][gitter-badge]][gitter-lobby]
  +[![Matrix][matrix-badge]][matrix-lobby]
  [![License][license-badge]][license-url]
  [actions-badge]: https://github.com/pikelet-lang/pikelet/workflows/ci/badge.svg
  [actions-url]: https://github.com/pikelet-lang/pikelet/actions
  -[gitter-badge]: https://badges.gitter.im/pikelet-lang/pikelet.svg
  -[gitter-lobby]: https://gitter.im/pikelet-lang/Lobby
  +[matrix-badge]: https://img.shields.io/matrix/pikelet:matrix.org?label=%23pikelet%3Amatrix.org
  +[matrix-lobby]: https://app.element.io/#/room/#pikelet:matrix.org
  [license-badge]: https://img.shields.io/github/license/pikelet-lang/pikelet
  [license-url]: ./LICENSE
  