Add changelog for named arguments (#16262)

changelog/dmd.named-arguments.dd

@@ -0,0 +1,41 @@
+Named arguments for functions have been implemented and documented
+
+When calling a function, arguments may be preceded with a parameter name for purposes of clarity and flexible ordering.
+Consequently, default arguments need not be at the end of the parameter list anymore.
+
+---
+void createWindow(bool fullScreen = false, int width, int height, string title);
+
+void main()
+{
+    createWindow(title: "Skynet", width: 1280, height: 720);
+}
+---
+
+Named arguments can also be used in struct/union literals.
+A union can now be initialized by setting a field different than the first one.
+
+---
+union U
+{
+    float asFloat;
+    uint asInt;
+}
+
+auto u0 = U(1.0); // this sets the `asFloat` field
+auto u1 = U(asInt: 0x3F800000); // formerly not possible
+---
+
+Relevant specification pages are:
+$(UL
+    $(LI $(DDSUBLINK spec/struct, struct-literal, Struct Literals))
+    $(LI $(DDSUBLINK spec/struct, anonymous, Anonymous Structs and Unions))
+    $(LI $(DDSUBLINK spec/expression, argument-parameter-matching, Matching Arguments to Parameters))
+    $(LI $(DDSUBLINK spec/function, function-overloading, Function Overloading))
+)
+
+Note that the implementation for regular functions and struct literals has been around since dmd 2.103, but it was undocumented and wouldn't work with template functions.
+
+This implements [DIP1030](https://www.dlang.org/dips/1030) for *function arguments*, but named *template arguments* are not implemented yet.
+Also, there are still implementation details to be ironed out which the DIP doesn't specify, such as how named arguments interact with tuples.
+For more information, see: [Named Arguments Status Update](https://forum.dlang.org/post/bynneksajyfyadwndsbm@forum.dlang.org)