[git] Add reference.md
[git] Add reference.md

git 참고자료 모음 문서 생성
git submodule 사용법 링크 추가

honux77/practice,honux77/practice,honux77/practice,honux77/practice,honux77/practice,honux77/practice,honux77/practice,honux77/practice,honux77/practice,honux77/practice,honux77/practice,honux77/practice,honux77/practice