docs(reactive): improved the vertx client section of getting-started-reactive

Author: hantsy

docs/src/main/asciidoc/getting-started-reactive.adoc

@@ -594,6 +594,25 @@ This resource returns `Uni` and `Multi` instances based on the result produced b
 
 The previous example uses a _service_ provided by Quarkus.
 Also, you can use Vert.x clients directly.
+
+First of all, make sure the `quarkus-vertx` extension is present. If not, activate the extension by executing the following command:
+
+[source,shell,subs=attributes+]
+----
+mvn io.quarkus:quarkus-maven-plugin:{quarkus-version}:add-extensions \
+    -Dextensions=vertx
+----
+
+Or add `quarkus-vertx` into your dependencies manually.
+
+[source, xml]
+----
+<dependency>
+	<groupId>io.quarkus</groupId>
+	<artifactId>quarkus-vertx</artifactId>
+</dependency>
+----
+
 There is a Mutiny version of the Vert.x APIs.
 This API is divided into several artifacts you can import independently: