C#: Add change note

Author: hvitved

csharp/change-notes/2021-05-03-implicit-constructor-init.md

@@ -0,0 +1,14 @@
+lgtm,codescanning
+* Implicit base constructor calls are now extracted. For example, in
+  ```csharp
+    class Base
+    {
+        public Base() { }
+    }
+
+    class Sub : Base
+    {
+        public Sub() { }
+    }
+  ```
+  there is an implicit call to the `Base` constructor from the `Sub` constructor.