Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

webapp/channels/CONTRIBUTING.md

@@ -0,0 +1,5 @@
+# Code Contribution Guidelines
+
+Thank you for your interest in contributing! Please see the [Mattermost Contribution Guide](https://developers.mattermost.com/contribute/getting-started/) which describes the process for making code contributions across Mattermost projects and [join our "Contributors" community channel](https://community.mattermost.com/core/channels/tickets) to ask questions from community members and the Mattermost core team.
+
+When you submit a pull request, it goes through a [code review process outlined here](https://developers.mattermost.com/contribute/getting-started/code-review/).