SL_3 'Do not add non-standard entities to namespace std' add example (#2031)

* SL_3 add example

* Update isocpp.dic

add My_Vector

CppCoreGuidelines.md

@@ -19754,7 +19754,17 @@ Additions to `std` might clash with future versions of the standard.
 
 ##### Example
 
-    ???
+    namespace std { // BAD: violates standard
+        class My_vector {
+        //     . . .
+        };
+    }
+
+    namespace Foo { // GOOD: user namespace is allowed 
+        class My_vector {
+        //     . . .
+        };
+    }
 
 ##### Enforcement