sudo -u git ssh-keygen -f /home/git/.ssh/reponame_key

Host reponame
IdentityFile /home/git/.ssh/reponame_key
HostName github.com
User git

cd /home/git/repositories/namespace/reponame.git

git remote add --mirror github reponame:youruser/reponame.git

cd /home/git/repositories/namespace/reponame.git/hooks

echo "exec git push --quiet github &" >> post-receive

chmod 755 post-receive