Add README to webapp folder (#22831)

* Add README to webapp folder

* Remove old web app README

* Rewrite a sentence to remove jargon

webapp/channels/README.md

@@ -1,56 +0,0 @@
-# Mattermost Web App
-# [![Mattermost](https://user-images.githubusercontent.com/7205829/137170381-fe86eef0-bccc-4fdd-8e92-b258884ebdd7.png)](https://mattermost.com)
-
-[Mattermost](https://mattermost.com) is an open source platform for secure collaboration across the entire software development lifecycle. This repo is the primary source for core development on the Mattermost platform; it's written in Golang and React, and runs as a single Linux binary with MySQL or PostgreSQL. A new compiled version is released under an MIT license every month on the 16th.
-
-<img width="1006" alt="mattermost-hero" src="https://user-images.githubusercontent.com/7205829/136107976-7a894c9e-290a-490d-8501-e5fdbfc3785a.png">
-
-This repo hosts the webapp client code. If you'd like to report an issue with Mattermost, please create an issue on [mattermost-server](https://github.com/mattermost/mattermost-server), which hosts the server code. You can also look at the [Get Help With Mattermost](https://docs.mattermost.com/guides/get-help.html) guide to find the resources available to our community.
-
-- [Product documentation](https://docs.mattermost.com/)
-- [Developer documentation](https://developers.mattermost.com/)
-- [Download compiled version](https://mattermost.com/download)
-
-## Try out Mattermost
-
-- [Join the Mattermost Contributor's server](https://community.mattermost.com/signup_user_complete/?id=codoy5s743rq5mk18i7u5ksz7e) to join community discussions about contributions, development and more
-- [Get started with Mattermost Cloud](https://customers.mattermost.com/cloud/signup) to try out Mattermost
-
-[![Deploy a Preview](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/mattermost/mattermost-heroku)
-
-_Note: Heroku preview does not include email or persistent storage._
-
-## Install Mattermost
-
-- [Deploy Guide](https://docs.mattermost.com/guides/deployment.html) - Deploy Mattermost in minutes via Docker, Ubuntu, or tar.
-- [Developer Machine Setup](https://developers.mattermost.com/contribute/server/developer-setup) - Follow this guide if you want to write code for Mattermost
-
-Other Install Guides:
-- [Deploy Mattermost on Docker](https://docs.mattermost.com/install/install-docker.html)
-- [Mattermost Omnibus](https://docs.mattermost.com/install/installing-mattermost-omnibus.html)
-- [Install Mattermost from Tar](https://docs.mattermost.com/install/install-tar.html)
-- [Ubuntu 20.04 LTS](https://docs.mattermost.com/install/installing-ubuntu-2004-LTS.html)
-- [Kubernetes](https://docs.mattermost.com/install/install-kubernetes.html)
-- [Helm](https://docs.mattermost.com/install/install-kubernetes.html#installing-the-operators-via-helm)
-- [Debian Buster](https://docs.mattermost.com/install/install-debian.html)
-- [RHEL 8](https://docs.mattermost.com/install/install-rhel-8.html)
-- [More server install guides](https://docs.mattermost.com/guides/deployment.html)
-
-## Get Involved
-
-- [Contribute to Mattermost](https://handbook.mattermost.com/contributors/contributors/ways-to-contribute)
-- [Find "Help Wanted" projects](https://github.com/mattermost/mattermost-server/issues?page=1&q=is%3Aissue+is%3Aopen+%22Help+Wanted%22&utf8=%E2%9C%93)
-- [Join Developer Discussion on a Mattermost Server for contributors](https://docs.mattermost.com/guides/community-chat.html)
-- [Get Help With Mattermost](https://docs.mattermost.com/guides/get-help.html)
-
-## Learn More
-
-- [API Options - webhooks, slash commands, drivers and web service](https://api.mattermost.com/)
-- [Localization Guide](https://handbook.mattermost.com/contributors/contributors/localization)
-
-## Get the Latest News
-
-- **Twitter** - Follow [Mattermost](https://twitter.com/Mattermost)
-- **Email** - Subscribe to our [newsletter](https://mattermost.com/community-newsletter/)
-
-Any other questions, mail us at [info@mattermost.com](mailto:info@mattermost.com). We'd love to meet you!