Update README.md

Author: rcalago

core-kotlin-modules/core-kotlin-collections-map/README.md

@@ -6,3 +6,4 @@
 - [Implement a MultiMap in Kotlin](https://www.baeldung.com/kotlin/multimap)
 - [Skip Null Values When Converting a List to a Map in Kotlin](https://www.baeldung.com/kotlin/list-map-conversion-ignore-nulls)
 - [Creating a Map from Two Arrays in Kotlin](https://www.baeldung.com/kotlin/map-from-two-arrays)
+- [Converting a List of Maps to a Map Grouped by Key in Kotlin](https://www.baeldung.com/kotlin/map-grouping)