Added gif for enhanced visual learning

slides/01-cpp.html

@@ -1423,7 +1423,9 @@ <h2>swap()</h2>
 ```
 - Use when formal parameter should be *able to change the value* of the actual argument
   - This is when references are used (and for return values); nobody bothers to declare one inside method
-</script></section>
+</script>
+<img class="stretch" alt="Difference between passing by reference vs value" src="images/01-cpp/pass_by_ref_value.gif">
+</section>
 
 <section data-markdown><script type="text/template">
 ## Call By Constant Reference