update readme

README.md

@@ -44,6 +44,17 @@ apt install git\
 
 ## Development
 
+### Structure
+
+Application sources located at `src/app` folder, by following basic principles:
+
+* Every file extends one of GTK 4 Widgets according to subject (e.g. `class Browser : public Gtk::ApplicationWindow`)
+* `src/app` does not contain other class entities like models or libraries (another folders and namespaces at `src` root are reserved for these needs)
+* Every file work with it own, 1th level child only, to prevent massive logic levels keeping in mind
+* To access any children features, deeper or higher than 1th level of current class, use delegation methods (actions, getters and setters)
+* One file - one class. If the file requires additional (GTK) component, this component should be placed at similar folder with same name as parent filename. So we have simple hierarchy navigation logic - from app to window, from window to it container, etc.
+* At this moment, all constants named with uppercase, const everything that not mutable
+
 ### Environment
 
 ``` bash