docs: added dependency graph to readme.md

.github/readme.md

@@ -13,3 +13,18 @@ Pure Dart, fully tested and ISO-driven ecosystem for the world's data in form of
 | [sealed_countries](https://github.com/tsinis/sealed_world/tree/main/packages/sealed_countries)   | Countries in form of compile-time constant sealed classes.                                               | [![pub.dev](https://img.shields.io/pub/v/sealed_countries.svg)](https://pub.dev/packages/sealed_countries)   |
 | [sealed_currencies](https://github.com/tsinis/sealed_world/tree/main/packages/sealed_currencies) | Currencies in form of compile-time constant sealed classes.                                              | [![pub.dev](https://img.shields.io/pub/v/sealed_currencies.svg)](https://pub.dev/packages/sealed_currencies) |
 | [sealed_languages](https://github.com/tsinis/sealed_world/tree/main/packages/sealed_languages)   | Languages in form of compile-time constant sealed classes.                                               | [![pub.dev](https://img.shields.io/pub/v/sealed_languages.svg)](https://pub.dev/packages/sealed_languages)   |
+
+```mermaid
+graph TD
+sealed_languages(sealed_languages)
+sealed_currencies(sealed_currencies)
+sealed_countries(sealed_countries)
+world_countries(world_countries)
+
+sealed_languages --> sealed_currencies --> sealed_countries --> world_countries
+
+click sealed_languages "https://github.com/tsinis/sealed_world/tree/main/packages/sealed_languages"
+click sealed_currencies "https://github.com/tsinis/sealed_world/tree/main/packages/sealed_currencies"
+click sealed_countries "https://github.com/tsinis/sealed_world/tree/main/packages/sealed_countries"
+click world_countries "https://github.com/tsinis/sealed_world/tree/main/packages/world_countries"
+```