! [rejected] master -> master (non-fast-forward)error: failed to push some refs to 'https://github.com/username/username.github.io.git'hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Merge the remote changes (e.g. 'git pull')hint: before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help'for details.