Extended TESTING.asciidoc with an example of running just the REST tests from maven

TESTING.asciidoc

@@ -180,10 +180,20 @@ https://github.com/elasticsearch/elasticsearch-rest-api-spec/tree/master/test[YA
 that describe the operations to be executed and the obtained results that
 need to be tested.
 
+The REST tests are run automatically when executing the maven test command. To run only the
+REST tests use the following command:
+
+---------------------------------------------------------------------------
+mvn test -Dtests.class=org.elasticsearch.test.rest.ElasticsearchRestTests
+---------------------------------------------------------------------------
+
 `ElasticsearchRestTests` is the executable test class that runs all the
 yaml suites available through a git submodule within the `rest-spec` folder.
 The submodule gets automatically initialized through maven right before
 running tests (generate-test-resources phase).
+
+To run just the RESTs test using maven, use the following command:
+
 The REST tests cannot be run without the files pulled from the submodule,
 thus if the `rest-spec` folder is empty on your working copy, it means
 that it needs to be initialized with the following command: