[chapter 06 #117] Introduce Explaining Variable: add readme
[chapter 06 #119] Introduce Explaining Variable: add example
[chapter 06 #119] Introduce Explaining Variable: assign variable to an expression, then replace the expression with the variable
[chapter 06 #119] Introduce Explaining Variable: refactored example with explaining variables
[chapter 06 #119] Introduce Explaining Variable: refactored example with extracting methods