Add a note to enable reliable websockets on plugin install (mattermos…

…t-community#1758)

* Add a note to enable reliable websockets on plugin install

* Update _index.md

Co-authored-by: Chen-I Lim <46905241+chenilim@users.noreply.github.com>

website/site/content/download/mattermost/latest-plugin/_index.md

@@ -25,3 +25,7 @@ Then upload the Focalboard plugin:
 ## Update your web proxy configuration
 
 Follow the [steps here](../#with-nginx) to configure your web proxy (NGINX, Apache, etc.) to complete the setup.
+
+## Enable Reliable Websockets
+
+The Focalboard plugin requires [the Reliable Websockets feature](https://docs.mattermost.com/configure/configuration-settings.html?highlight=enablereliablewebsockets#enable-reliable-websockets) to be enabled. Check in the Mattermost server configuration that the property `ServiceSettings.EnableReliableWebSockets` is set to `true`. If it's not, enable it, restart the server and reload the clients before using the Focalboard plugin.