Add missing link to app.asciidoc in guide index

doc/src/guide/book.asciidoc

@@ -10,6 +10,10 @@ include::overview.asciidoc[Overview]
 
 include::updating.asciidoc[Updating Erlang.mk]
 
+= Code
+
+include::app.asciidoc[Building]
+
 = Advanced
 
 include::external_plugins.asciidoc[External plugins]