Fix #493 array myth

Added comment about performance to SL.con.1

CppCoreGuidelines.md

@@ -1,6 +1,6 @@
 # <a name="main"></a>C++ Core Guidelines
 
-December 26, 2017
+Janualy 1, 2018
 
 
 Editors:
@@ -18558,6 +18558,13 @@ For a variable-length array, use `std::vector`, which additionally can change it
 
 Use `gsl::span` for non-owning references into a container.
 
+##### Note
+
+Comparing the performance of a fixed-sized array allocated on the stack against a `vector` with its elements on the free store is bogus.
+You could just as well compare a `std::array` on the stack against the result of a `malloc()` accessed through a pointer.
+For most code, even the difference between stack allocation and free-store allocation doesn't matter, but the convenieance and safety of `vector` does.
+People working with code for which that difference matters are quite capable of choosing between `array` and `vector`.
+
 ##### Enforcement
 
 * Flag declaration of a C array inside a function or class that also declares an STL container (to avoid excessive noisy warnings on legacy non-STL code). To fix: At least change the C array to a `std::array`.