message: Prepare changelog for v0.12.6
diff: +Changes in [0.12.6](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.6) (2018-05-25)
  +=====================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.6-rc.1...v0.12.6)
  +
  + * No changes since v0.12.6-rc.1
  +
  Changes in [0.12.6-rc.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.6-rc.1) (2018-05-24)
  ===============================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.5...v0.12.6-rc.1)
  