[HttpClient] Issue notice when NativeHttpClient is used

src/Symfony/Component/HttpClient/HttpClient.php

@@ -61,6 +61,8 @@ final class HttpClient
             return new AmpHttpClient($defaultOptions, null, $maxHostConnections, $maxPendingPushes);
         }
 
+        @trigger_error((\extension_loaded('curl') ? 'Upgrade' : 'Install').' the curl extension or run "composer require amphp/http-client" to perform async HTTP operations, including full HTTP/2 support', E_USER_NOTICE);
+
         return new NativeHttpClient($defaultOptions, $maxHostConnections);
     }