Mention performance improvement to Set and Map in Release Notes (#10425)

* Mention performance improvement to Set and Map in Release Notes

Mention performance improvement to Set and Map in Release Notes
see https://twitter.com/buybackoff/status/1326530575902011392

* Avoid superlatives

Co-authored-by: Phillip Carter <pcarter@fastmail.com>

Co-authored-by: Phillip Carter <pcarter@fastmail.com>

Author: goswinr

release-notes.md

@@ -32,6 +32,7 @@ This release covers three important milestones: F# 5, Visual Studio 16.8, and .N
 
 ### FSharp Core 5.0.0
 
+* [Performance improvement](https://github.com/dotnet/fsharp/pull/10188) to core collections Set and Map by [Victor Baybekov](https://github.com/buybackoff) 
 * Consistent behavior for empty/non-existent slices for lists, strings, arrays, 2D arrays, 3D arrays, and 4D arrays
 * Support for fixed-index slices in 3D and 4D arrays
 * Support for negative indexes (in preview)