message: Prepare changelog for v0.12.0-rc.6
diff: +Changes in [0.12.0-rc.6](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0-rc.6) (2018-04-09)
  +===============================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0-rc.5...v0.12.0-rc.6)
  +
  + * Fix group join button not appearing
  +
  Changes in [0.12.0-rc.5](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0-rc.5) (2018-04-09)
  ===============================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0-rc.4...v0.12.0-rc.5)
  