Add additional source code creation rules

doc/coding-guidelines.md

@@ -33,3 +33,4 @@ The following naming conventions must be met for code contribution:
 The following source code creation guidelines must be followed:
 
 1. In source code (.cpp files) namespaces must not be used throughout the code and must be announced through the __using__ keyword after the imports (e.g. using ls::std::boxing::Integer). The only exception is when the same class name is being used in more than one namespace.
+2. Include directives in header files must be used with angle brackets. Include paths are handled via CMake. The only exception to that rule is a header file, which is located in the same directory. On the contrary, include directives in source files must always be in angle brackets writing.