Add attributes ordering to the DStyle

dstyle.dd

@@ -342,6 +342,7 @@ $(LISTSECTION Attributes,
          matching attributes (`@nogc`, `@safe`, `pure`, `nothrow`).)
     $(LI $(I Templated) functions should $(B not) be annotated with attributes
          as the compiler can infer them.)
+    $(LI Attributes should be listed in alphabetical ordering, e.g. `const @nogc nothrow pure @safe`)
     $(LI $(B Every) $(I unittest) should be annotated
          (e.g. `pure nothrow @nogc @safe unittest { ... }`)
          to ensure the existence of attributes on the templated function.)