Attempts to make corrections about Exercise 4.31

There is no different between prefix and postfix in this example. :heavy_division_sign:
Create README.md of chapter 5 and add ex5.1-5.3
Added ex 5.4
Create ex5_5.cpp
Create ex5_6.cpp
added 5.5~5.8
Update README.md
Create ex5_9.cpp
Fixed print hint. (adding the capital letters)
Update ex5_10.cpp

The switch statemen more better.
Create ex5_11.cpp
Create ex5_12.cpp
added 5.9~5.13
Update ex5_14.cpp
added 5.14
Fixed little bug.

The program will print wrong hint when there is **no duplicated** words in user's inputs.
Added 5.15~5.17
Create ex5_17.cpp
added 5.18~5.19
Create ex5_19.cpp