Update README.md

Author: rcalago

core-kotlin-modules/core-kotlin-strings-5/README.md

@@ -6,4 +6,5 @@ This module contains articles about core Kotlin strings.
 
 - [Extract Numbers From a String in Kotlin](https://www.baeldung.com/kotlin/string-extract-numeric-value)
 - [Parse String With Thousands Separator in Kotlin](https://www.baeldung.com/kotlin/string-parse-thousands-separator)
+- [Guide to the HexFormat API](https://www.baeldung.com/kotlin/hexadecimal-string-parse)
 - - More articles: [[<-- prev]](../core-kotlin-strings-4)