Add section about "Deeper shell integration"
Add section about "Deeper shell integration"

Also adds a snippet for zsh user to auto-issue `nvm use` when entering a
directory.

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