Add warning about plugin permission (#4383)

* Add warning about plugin permission

I didn't see any documentation that described what an installed plugin can do - and it would be good to make sure cluster operators have expectations described somewhere.

* Update plugins.md

* Update _install-and-configure/plugins.md

Co-authored-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>
Signed-off-by: Peter Nied <peternied@hotmail.com>

* Update _install-and-configure/plugins.md

Co-authored-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>
Signed-off-by: Peter Nied <peternied@hotmail.com>

* Incorperate cwillum's feedback

Signed-off-by: Peter Nied <peternied@hotmail.com>
Co-authored-by: cwillum <cwmmoore@amazon.com>

---------

Signed-off-by: Peter Nied <peternied@hotmail.com>
Co-authored-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>
Co-authored-by: cwillum <cwmmoore@amazon.com>

_install-and-configure/plugins.md

@@ -11,6 +11,10 @@ redirect_from:
 
 You can install individual plugins for OpenSearch based on your needs. For information about available plugins, see [Available plugins](#available-plugins).
 
+
+For plugins to work properly with OpenSearch, all plugins must have the ability to access the data in the cluster, including metadata about cluster operations. Therefore, to protect your cluster's data and preserve its integrity, first be sure you understand the function of a plugin before installing it on your OpenSearch cluster. Second, when selecting a custom plugin, make sure the plugin's source is a reliable one.
+{: .warning}
+
 ## Managing plugins
 
 OpenSearch uses a command line tool called `opensearch-plugin` for managing plugins. This tool allows you to: