osmotrx: Introduce code architecture chapter

Change-Id: I21084e6315d79a1adcb305e12343da218837dc31
diff --git a/doc/manuals/osmotrx-usermanual.adoc b/doc/manuals/osmotrx-usermanual.adoc
index a713e4a..14f5514 100644
--- a/doc/manuals/osmotrx-usermanual.adoc
+++ b/doc/manuals/osmotrx-usermanual.adoc
@@ -29,6 +29,8 @@
 
 include::chapters/trx-backends.adoc[]
 
+include::chapters/code-architecture.adoc[]
+
 include::../common/chapters/trx_if.adoc[]
 
 include::../common/chapters/port_numbers.adoc[]