C#: Add change note

Author: hvitved

csharp/change-notes/2020-11-05-get-sourcedeclaration-rename.md

@@ -0,0 +1,4 @@
+lgtm,codescanning
+* The predicate `Declaration::getSourceDeclaration/0` has been renamed to `Declaration::getUnboundDeclaration/0`.
+  This is to avoid confusion, as it has nothing to do with declarations from source code (as opposed to declarations
+  in assemblies), but instead has to do with generics.