Update README.md

Author: rcalago

core-kotlin-modules/core-kotlin-strings-4/README.md

@@ -13,3 +13,4 @@ This module contains articles about core Kotlin strings.
 - [Sorting a String Alphabetically in Kotlin](https://www.baeldung.com/kotlin/string-sort)
 - [Kotlin Equivalent of Java’s equalsIgnoreCase()](https://www.baeldung.com/kotlin/java-equalsignorecase-equivalent)
 - [Convert String to Char in Kotlin](https://www.baeldung.com/kotlin/string-char-conversion)
+- [Java Boolean.valueOf() Equivalent in Kotlin](https://www.baeldung.com/kotlin/boolean-valueof-equivalent)