add example to R.20

addresses #1015

CppCoreGuidelines.md

@@ -1,6 +1,6 @@
 # <a name="main"></a>C++ Core Guidelines
 
-August 11, 2017
+September 17, 2017
 
 
 Editors:
@@ -9103,6 +9103,8 @@ Consider:
         X* p1 { new X };              // see also ???
         unique_ptr<T> p2 { new X };   // unique ownership; see also ???
         shared_ptr<T> p3 { new X };   // shared ownership; see also ???
+        auto p4 = make_unique<X>();   // unique_ownership, preferable to the explicit use "new"
+        auto p5 = make_shared<X>();   // shared ownership, preferable to the explicit use "new"
     }
 
 This will leak the object used to initialize `p1` (only).