Allow custom snippets directory via JUnit 5

See gh-633

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

@@ -397,6 +397,18 @@ based on your project's build tool:
 
 |===
 
+If you are using JUnit 5.1 or later the default can be overridden by providing
+an output directory when registering the `RestDocumentationExtension` instance:
+
+[source,java,indent=0]
+----
+public class JUnit5ExampleTests {
+    @RegisterExtension
+    static final RestDocumentationExtension restDocumentation =
+        new RestDocumentationExtension ("custom");
+}
+----
+
 
 
 Next, you must provide a `@BeforeEach` method to configure MockMvc, WebTestClient, or