Update oh-my-zsh instructions in readme
Update oh-my-zsh instructions in readme

Oh-my-zsh overrides PS1 even when no theme is set, this commit adds a
note to the readme about the need to activate Pure *after* oh-my-zsh.

Closes #315.

sindresorhus/pure,joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles,supercrabtree/pure,jaredru/zsh-pure,forivall/pure,kgrz/pure,joshuarubin/dotfiles,mafredri/pure