adding the move assignment operator example

hws/matrix_class/optimization/README.md

@@ -0,0 +1,40 @@
+Compile and Run these two programs to see the performance difference, note that the two programs have the same main function.
+
+Here are the results:
+
+```console
+$ g++ matrix_fast.cpp -o matrix_fast
+$ g++ matrix_slow.cpp -o matrix_slow
+$ time ./matrix_fast
+
+real    0m1.134s
+user    0m1.130s
+sys     0m0.004s
+$ time ./matrix_fast
+
+real    0m1.138s
+user    0m1.134s
+sys     0m0.004s
+$ time ./matrix_fast
+
+real    0m1.139s
+user    0m1.139s
+sys     0m0.000s
+$ time ./matrix_slow
+
+real    0m2.159s
+user    0m2.159s
+sys     0m0.000s
+$ time ./matrix_slow
+
+real    0m2.161s
+user    0m2.157s
+sys     0m0.004s
+$ time ./matrix_slow
+
+real    0m2.207s
+user    0m2.200s
+sys     0m0.000s
+```
+
+As can be seen, the fast version is much faster than the slow version, this is because it avoids many memory allocations/deallocations.