Update fleet-agent-proxy-package-registry.asciidoc

Author: lucabelluccini

docs/en/ingest-management/fleet-agent-proxy-package-registry.asciidoc

@@ -17,6 +17,10 @@ example:
 xpack.fleet.registryProxyUrl: your-nat-gateway.corp.net
 ----
 
+If your HTTP Proxy requires authentication, you can include the 
+credentials in the URI, such as `https://username:[email protected]`, 
+only when using HTTPS.
+
 == What information is sent to the {package-registry}?
 
 In production environments, {kib}, through the {fleet} plugin, is the only service interacting with the {package-registry}. Communication happens when interacting with the Integrations UI, and when upgrading {kib}. The shared information is about discovery of Elastic packages and their available versions. In general, the only deployment-specific data that is shared is the {kib} version.