refactor 10.18~19, we don't need stable_sort if we don't use find_if.
refactor 10.21
added 10.22 ~ 10.25 to README

1. added 10.22
2. move 10.23 from ex10_20_21_22_23.cpp
3. use const (10.24)
4. refactor 10.25
fixed links