hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.
github repository를 생성할 때 readme.md 생성으로 인한 오류라고 한다.
pull해도 또 다른 오류가 발생한다.
+를 이용하여 에러 상관없이 강제로 push 하게 도와준다.
git push -u [origin] +master