Add documentation about variable substitutions

Author: urfeex

doc/architecture/script_reader.rst

@@ -80,6 +80,30 @@ The data dictionary supports the following types
 Script code side
 ~~~~~~~~~~~~~~~~
 
+Variable replacements
+^^^^^^^^^^^^^^^^^^^^^
+
+Variable replacements in the script code are done using the syntax ``{{ variable_name }}``. For
+this to work, the variable ``variable_name`` has to be defined in the data dictionary passed to the
+``ScriptReader``.
+
+The expression ``{{ variable_name }}`` will be replaced with the string representation of the
+variable's content.
+
+- If the variable is a string, it has to be wrapped into quotes in the script
+  code. e.g.
+
+  .. code-block:: python
+
+     textmsg("{{ log_message }}")
+
+
+- Boolean variables will be replaced with the string ``True`` or ``False``.
+- Numeric variables (integer and floating point) will be replaced with the string representation generated by
+  `std::to_string() <https://en.cppreference.com/w/cpp/string/basic_string/to_string>`_.
+- Version information variables will be replaced with the string representation similar to
+  ``10.7.0.0``
+
 Boolean expressions
 ^^^^^^^^^^^^^^^^^^^