Update README.md

Author: amnabibi786

kotlin-libraries-data/README.md

@@ -5,3 +5,4 @@ This module contains articles about data manipulation libraries topics in Kotlin
 - [Class Inheritance with Kotlinx Serialization](https://www.baeldung.com/kotlin/kotlinx-serialization-inheritance)
 - [Mapstruct With Kotlin’s Data Classes](https://www.baeldung.com/kotlin/mapstruct-data-classes)
 - [Deserialize Enum While Ignoring Unknown Values in Kotlin](https://www.baeldung.com/kotlin/unknown-enums-deserialization)
+- [Serialize Enum in Retrofit With Kotlin](https://www.baeldung.com/kotlin/retrofit-enum)