Add zshrc hook for private Nutshell-specific config

zsh/.nutshell.zshrc should not have any additional changes applied to it
-- it's merely a placeholder. Therefore after this commit was applied I
ran this command:

    git update-index --assume-unchanged zsh/.nutshell.zshrc

If I really do need to make changes, I'll need to undo this
assume-unchanged flag:

    git update-index --no-assume-unchanged zsh/.nutshell.zshrc