Add C# notes

Co-authored-by: Adam Reeve <[email protected]>

_posts/2024-07-16-17.0.0-release.md

@@ -143,6 +143,12 @@ Thanks for your contributions and participation in the project!
 
 ## C# notes
 
+- The performance of building Decimal arrays using SqlDecimal values was improved for .NET 7+ (GH-41349)
+- Scalar arrays now implement `ICollection<T?>` (GH-38692)
+- Concatenating arrays with a non-zero offset with ArrowArrayConcatenator was fixed (GH-41164)
+- Concatenating union arrays with ArrowArrayConcatenator was fixed (GH-41198)
+- Accessing values of decimal arrays with a non-zero offset was fixed (GH-41199)
+
 ## Go Notes
 
 ## Java notes