[readme] Auto install not exist version in zsh .nvmrc script
[readme] Auto install not exist version in zsh .nvmrc script

Fixes #1272.

edwmurph/nvm,PeterDaveHello/nvm,edwmurph/nvm,evocateur/nvm,PeterDaveHelloKitchen/nvm,ljharb/nvm,creationix/nvm,PeterDaveHello/nvm,creationix/nvm,PeterDaveHelloKitchen/nvm,evocateur/nvm,ljharb/nvm