docs/userguide: document for_window (Thanks aksr)

docs/userguide

@@ -434,6 +434,31 @@ new_window <normal|1pixel|borderless>
 new_window 1pixel
 ---------------------
 
+=== Arbitrary commands for specific windows (for_window)
+
+With the +for_window+ command, you can let i3 execute any command when it
+encounters a specific window. This can be used to set windows to floating or to
+change their border style, for example.
+
+*Syntax*:
+-----------------------------
+for_window [criteria] command
+-----------------------------
+
+*Examples*:
+------------------------------------------------
+# enable floating mode for all XTerm windows
+for_window [class="XTerm"] mode floating
+
+# Make all urxvts use a 1-pixel border:
+for_window [class="urxvt"] border 1pixel
+
+# A less useful, but rather funny example:
+# makes the window floating as soon as I change
+# directory to ~/work
+for_window [title="x200: ~/work"] mode floating
+------------------------------------------------
+
 === Variables
 
 As you learned in the section about keyboard bindings, you will have