Updates to documentation (#280)

Documentation updates (including moving of header files

lib_xua/doc/rst/add.rst

@@ -0,0 +1,27 @@
+
+Additional Features
+-------------------
+
+The previous sections describes only the basic core set of ``lib_xua`` details on enabling additional features e.g. S/PDIF are discussed in this section.
+
+Where something must be defined, it is recommended this is done in `xua_conf.h` but could also be done in the application Makefile.
+
+For each feature steps are listed for if calling ``lib_xua`` functions manually - if using the "codeless" programming model then these steps are informational only. 
+Each section also includes a sub-section on enabling the feature using the "codeless" model.
+
+For full details of all options please see the API section
+
+
+.. toctree::
+
+    S/PDIF Transmit <feat_spdif_tx>
+
+
+
+
+
+
+
+
+
+