https://matrix.org/

https://matrix.org/faq/

https://matrix.org/docs/guides/introduction

integration
    https://github.com/mautrix/telegram
    https://github.com/Half-Shot/matrix-appservice-discord
    https://github.com/matrix-org/matrix-appservice-slack

whatsnew
https://matrix.org/blog/2019/06/11/introducing-matrix-1-0-and-the-matrix-org-foundation