Add note about product_query default constructor

Author: knoepfel

phlex/core/product_query.hpp

@@ -12,6 +12,8 @@
 namespace phlex::experimental {
   class product_query {
   public:
+    // FIXME: Boost JSON's parameter retrieval facilities require a default constructor
+    //        whenever the type is (e.g.) std::array<product_query< N>.
     product_query();
     product_query(product_specification spec, std::string layer);