Information for programmer in documation

doc/index.rst

@@ -20,6 +20,7 @@ Contents:
     :maxdepth: 2
 
     user
+    programmer
 
 .. _Yaml: http://yaml.org
 

doc/programmer.rst

@@ -0,0 +1,40 @@
+
+================
+Programmer Guide
+================
+
+As a programmer you are looking for source code.
+You found it, here it is.
+
+.. literalinclude:: ./cfgdemo/src/main.rs
+    :language: rust
+
+
+Key link
+========
+
+The key link between configuration file
+and the executable programm that you are writen
+is the configuration ``struct``.
+
+In the demo source code is struct named *Config*
+and is *cfg* a variable.
+
+
+config.yaml
+===========
+
+Nothing of ``Config`` or ``cfg`` needs to be in the YAML.
+But the field names **must**.
+
+Here the *.yaml* that goes with the above example source code.
+
+
+.. literalinclude:: ./cfgdemo/config.yaml
+    :language: yaml
+
+
+See also
+========
+
+The documentation of `serde <https://serde.rs/>`_.