Add documentation on remote debugging

For a node which is launched outside the context of Gradle.

TESTING.asciidoc

@@ -474,7 +474,7 @@ Combined (Unit+Integration) coverage:
 mvn -Dtests.coverage verify jacoco:report
 ---------------------------------------------------------------------------
 
-== Debugging from an IDE
+== Launching and debugging from an IDE
 
 If you want to run elasticsearch from your IDE, the `gradle run` task
 supports a remote debugging option:
@@ -483,6 +483,23 @@ supports a remote debugging option:
 gradle run --debug-jvm
 ---------------------------------------------------------------------------
 
+== Debugging remotely from an IDE
+
+If you want to run elasticsearch and be able to remotely attach the process
+for debugging purposes from your IDE, you need to add the following line in
+`config/jvm.options`:
+
+---------------------------------------------------------------------------
+-Xdebug -Xrunjdwp:server=y,transport=dt_socket,address=4000,suspend=n
+---------------------------------------------------------------------------
+
+Then start elasticsearch with `bin/elasticsearch` as usual.
+
+If you are using IntelliJ, create a new Run/Debug configuration, choose `Remote`
+and define the same port you defined in `config/jvm.options`. Then start the
+remote debug session from IntelliJ.
+
+
 == Building with extra plugins
 Additional plugins may be built alongside elasticsearch, where their
 dependency on elasticsearch will be substituted with the local elasticsearch