[Docs] Explain `.nvmrc` better
[Docs] Explain `.nvmrc` better

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