Added text about git submodule

README.md

@@ -4,6 +4,12 @@
 Tablexia is a modern educational application aiming to support the development of cognitive abilities. It is designed primarily for children with dyslexia in secondary schools. It should find its use in schools to supplement the standard teaching, as well as in pedagogical-psychological counseling offices and other counseling facilities for pupils with learning difficulties. 
 
 ## <b>Build</b>
+
+Project is working with git submodule Tablexia-model. Before building the application, it needs to be called:
+
+`git submodule update --init --recursive`
+
+<br />
 Tablexia can be build by running the command:
 
 `./gradlew build`