Update README.md

Author: malczuuu

README.md

@@ -15,7 +15,7 @@ It is intended to be used as a **foundation** for other libraries or application
 - [Features](#features)
 - [Example](#example)
 - [Usage](#usage)
-- [Other Libraries](#other-libraries)
+- [Problem4J Links](#problem4j-links)
 
 ## Features
 
@@ -35,8 +35,8 @@ It is intended to be used as a **foundation** for other libraries or application
 ## Example
 
 ```java
-import io.github.malczuuu.problem4j.Problem;
-import io.github.malczuuu.problem4j.ProblemException;
+import io.github.malczuuu.problem4j.core.Problem;
+import io.github.malczuuu.problem4j.core.ProblemException;
 
 public class ExampleClass {
 
@@ -57,7 +57,8 @@ public class ExampleClass {
 
 ## Usage
 
-Add library as dependency to Maven or Gradle. See the actual versions on [Maven Central][maven-central]. **Java 8** or higher is required to use this library.
+Add library as dependency to Maven or Gradle. See the actual versions on [Maven Central][maven-central]. **Java 8** or
+higher is required to use this library.
 
 1. Maven:
    ```xml
@@ -76,8 +77,10 @@ Add library as dependency to Maven or Gradle. See the actual versions on [Maven
    }
     ```
 
-## Other Libraries
+## Problem4J Links
 
+- [`problem4j`][problem4j] - Documentation repository.
+- [`problem4j-core`][problem4j-core] - Core library defining `Problem` model and `ProblemException`.
 - [`problem4j-jackson`][problem4j-jackson] - Jackson module for serializing and deserializing `Problem` objects.
 - [`problem4j-spring-web`][problem4j-spring-web] - Spring Web module extending `ResponseEntityExceptionHandler` for
   handling exceptions and returning `Problem` responses.
@@ -86,6 +89,8 @@ Add library as dependency to Maven or Gradle. See the actual versions on [Maven
 
 [problem4j]: https://github.com/malczuuu/problem4j
 
+[problem4j-core]: https://github.com/malczuuu/problem4j-core
+
 [problem4j-jackson]: https://github.com/malczuuu/problem4j-jackson
 
 [problem4j-spring-web]: https://github.com/malczuuu/problem4j-spring-web