Build Gerrit in headless mode

  buck build headless

produces binary artifact without GWT.

Change-Id: Ic98312e9924d32057d6876effbdf12e489d44a8b
diff --git a/Documentation/dev-buck.txt b/Documentation/dev-buck.txt
index 6bd3905..c234651 100644
--- a/Documentation/dev-buck.txt
+++ b/Documentation/dev-buck.txt
@@ -116,6 +116,20 @@
 ----
 
 
+=== Headless Mode
+
+To build Gerrit in headless mode, i.e. without the GWT Web UI:
+
+----
+  buck build headless
+----
+
+The output executable WAR will be placed in:
+
+----
+  buck-out/gen/headless.war
+----
+
 === Extension and Plugin API JAR Files
 
 To build the extension, plugin and GWT API JAR files: