Add notes about user install and prezto to readme
Add notes about user install and prezto to readme

yuya373/pure,christopherscott/pure,bramkok/purist,anandubajith/pure,jaredru/zsh-pure,btaidm/pure,joshuarubin/dotfiles,fvargas/pure,KarlKode/pure,forivall/pure,sleep/pure,kgrz/pure,brandon099/pure,mcallan83/pure-plus,supercrabtree/pure,joshuarubin/dotfiles,dmiedema/pure,depressiveRobot/pure-time,boushley/pure,mafredri/pure,alem0lars/pure,kane-c/pure,DFurnes/purer,lavelle/pure,joshuarubin/dotfiles,enrdstrm/pure,sindresorhus/pure,jedahan/pure,perks/pure,ZefQ/pure,joshuarubin/dotfiles,runningskull/pure,rfnash/pure,estoner/pure