update readme

README.md

@@ -62,6 +62,7 @@ Currently, main application sources located at `src/app` folder, written by foll
 * At this moment, all constants named with uppercase, const everything that not mutable
 * `#include` application `.hpp` files in `.cpp`. For system libraries, use headers only. Do not place system dependencies in `.cpp`
 * App interact SQLite database, where table name match related class namespace, it may contain optional feature postfix (separated with double line)
+* Database class tables must contain only members of class and may include primary keys to their parent class tables, according to the app hierarchy
 
 ### Environment