Add comment per @knoepfel

See #121 (comment)

Co-authored-by: Kyle Knoepfel <knoepfel@fnal.gov>

Author: greenc-FNAL

phlex/model/product_store.hpp

@@ -77,7 +77,7 @@ namespace phlex::experimental {
     product_store_const_ptr parent_{nullptr};
     products products_{};
     level_id_ptr id_;
-    std::string source_;
+    std::string source_;  // FIXME: Should not have to copy the string (the source should outlive the product store)
     stage stage_;
   };