Homework2 readme suggestion

[AemonJape]

The encrypted binary output and diff were not readable at all.

Students would have a much easier time debugging if some formatting commands were suggested, such as the below.

./aes crack test/key.txt test/iv.txt test/helloworld.expected | xxd
diff <(xxd test/helloworld.txt) <(xxd _build/default/test/crack1.output)

It could also be added to the test output format.

[KihongHeo]

Thank you so much.

TAs, plz update the README.