Add accessibility for README file (mattermost#25206)

Co-authored-by: Jason Blais <13119842+jasonblais@users.noreply.github.com>

README.md

@@ -1,10 +1,10 @@
-# [![Mattermost](https://user-images.githubusercontent.com/7205829/137170381-fe86eef0-bccc-4fdd-8e92-b258884ebdd7.png)](https://mattermost.com)
+# [![Mattermost logo](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 Go 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.
 
 [Deploy Mattermost on-premises](https://mattermost.com/deploy/?utm_source=github-mattermost-server-readme), or [try it for free in the cloud](https://mattermost.com/sign-up/?utm_source=github-mattermost-server-readme).
 
-<img width="1006" alt="mattermost-hero" src="https://user-images.githubusercontent.com/7205829/136107976-7a894c9e-290a-490d-8501-e5fdbfc3785a.png">
+<img width="1006" alt="mattermost user interface" src="https://user-images.githubusercontent.com/7205829/136107976-7a894c9e-290a-490d-8501-e5fdbfc3785a.png">
 
 Learn more about the following use cases with Mattermost:
 
@@ -53,7 +53,7 @@ Other install guides:
 
 In addition to the web interface, you can also download Mattermost clients for [Android](https://mattermost.com/pl/android-app/), [iOS](https://mattermost.com/pl/ios-app/), [Windows PC](https://docs.mattermost.com/install/desktop-app-install.html#windows-10-windows-8-1), [macOS](https://docs.mattermost.com/install/desktop-app-install.html#macos-10-9), and [Linux](https://docs.mattermost.com/install/desktop-app-install.html#linux).
 
-[<img src="https://user-images.githubusercontent.com/30978331/272826427-6200c98f-7319-42c3-86d4-0b33ae99e01a.png" alt="Google Play" height="50px"/>](https://mattermost.com/pl/android-app/)  [<img src="https://developer.apple.com/assets/elements/badges/download-on-the-app-store.svg" alt="App Store" height="50px"/>](https://itunes.apple.com/us/app/mattermost/id1257222717?mt=8)  [![Windows PC](https://user-images.githubusercontent.com/33878967/33095357-39cab8d2-ceb8-11e7-89a6-67dccc571ca3.png)](https://docs.mattermost.com/install/desktop.html#windows-10-windows-8-1-windows-7)  [![Mac OSX](https://user-images.githubusercontent.com/33878967/33095355-39a36f2a-ceb8-11e7-9b33-73d4f6d5d6c1.png)](https://docs.mattermost.com/install/desktop.html#macos-10-9)  [![Linux](https://user-images.githubusercontent.com/33878967/33095354-3990e256-ceb8-11e7-965d-b00a16e578de.png)](https://docs.mattermost.com/install/desktop.html#linux)
+[<img src="https://user-images.githubusercontent.com/30978331/272826427-6200c98f-7319-42c3-86d4-0b33ae99e01a.png" alt="Get Mattermost on Google Play" height="50px"/>](https://mattermost.com/pl/android-app/)  [<img src="https://developer.apple.com/assets/elements/badges/download-on-the-app-store.svg" alt="Get Mattermost on the App Store" height="50px"/>](https://itunes.apple.com/us/app/mattermost/id1257222717?mt=8)  [![Get Mattermost on Windows PC](https://user-images.githubusercontent.com/33878967/33095357-39cab8d2-ceb8-11e7-89a6-67dccc571ca3.png)](https://docs.mattermost.com/install/desktop.html#windows-10-windows-8-1-windows-7)  [![Get Mattermost on Mac OSX](https://user-images.githubusercontent.com/33878967/33095355-39a36f2a-ceb8-11e7-9b33-73d4f6d5d6c1.png)](https://docs.mattermost.com/install/desktop.html#macos-10-9)  [![Get Mattermost on Linux](https://user-images.githubusercontent.com/33878967/33095354-3990e256-ceb8-11e7-965d-b00a16e578de.png)](https://docs.mattermost.com/install/desktop.html#linux)
 
 ## Get security bulletins
 
@@ -80,10 +80,10 @@ See the [LICENSE file](LICENSE.txt) for license rights and limitations.
 
 ## Get the latest news
 
-- **Twitter** - Follow [Mattermost](https://twitter.com/mattermost).
+- **X** - Follow [Mattermost on X, formerly Twitter](https://twitter.com/mattermost).
 - **Blog** - Get the latest updates from the [Mattermost blog](https://mattermost.com/blog/).
-- **Facebook** - Follow [Mattermost](https://www.facebook.com/MattermostHQ).
-- **LinkedIn** - Follow [Mattermost](https://www.linkedin.com/company/mattermost/).
+- **Facebook** - Follow [Mattermost on Facebook](https://www.facebook.com/MattermostHQ).
+- **LinkedIn** - Follow [Mattermost on LinkedIn](https://www.linkedin.com/company/mattermost/).
 - **Email** - Subscribe to our [newsletter](https://mattermost.us11.list-manage.com/subscribe?u=6cdba22349ae374e188e7ab8e&id=2add1c8034) (1 or 2 per month).
 - **Mattermost** - Join the ~contributors channel on [the Mattermost Community Server](https://community.mattermost.com).
 - **IRC** - Join the #matterbridge channel on [Freenode](https://freenode.net/) (thanks to [matterircd](https://github.com/42wim/matterircd)).