Update README.md for error corrections and more clarity

Hello Fabien, I like this kata, it includes a lot of bad taste and poor code :) I've changed a few things in the readme, and I think there is a need for some (optional) hints on how to test it; Legacy refactoring requires some tests, unless we decide to do a "scratch refactoring" session (no commit at the end).

My gut feeling is that is large enough to take much more than 2 hours to do fully (tests + refactoring). Different pairs could decide to tackle it in different ways, then compare their approaches.