Add a paragraph about smart pointers efficiency, citing the reference given in MR29.

As proposed in !29 (closed), add a paragraph about overhead in memory and performance for smart pointers, which is not paraphrasing the cited article.

5-UsefulConceptsAndSTL/6-SmartPointers.ipynb

 This simplifies the reading, especially if templates are also involved...
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+## The cost of using smart pointers
+
+This [article](https://www.modernescpp.com/index.php/memory-and-performance-overhead-of-smart-pointer) provides an analysis of the cost involved both in performance and memory when using a smart pointer. To put in the nutshell, its conclusions are:
+
+- `std::unique_ptr` bears no overhead in memory (except in a very edge case you can safely ignore for now) and little overhead in performance, at least for the latter when compiler optimizations are enabled (we will talk about them in a later [notebook](../6-InRealEnvironment/3-Compilers.ipynb)).
+- `std::shared_ptr` incurs a memory overhead (to keep the reference count) and a performance overhead as well (which is partially mitigated if the memory was allocated through `std::make_shared`). The performance overhead is especially important when no optimizations are involved.
+
+This highlights once more what we said earlier:
+
+- Use smart pointers: RAII is simply too precious to manage properly your ressources!
+- Use `std::unique_ptr` wherever you can, and use `std::shared_ptr` when you really need several instances of the same ressource.
+%% Cell type:markdown id: tags: