Update README.md

[skip ci]

Author: edizor

scala-core-collection-2/README.md

@@ -4,3 +4,4 @@
 - [Get a List Item by Index in Scala](https://www.baeldung.com/scala/list-get-item-by-index)
 - [Guide to Scala ListSet](https://www.baeldung.com/scala/listset)
 - [Guide to Scala ListMap](https://www.baeldung.com/scala/listmap)
+- [How to Flatten Collections in Scala](https://www.baeldung.com/scala/flattening-collections)