Updated README
Updated README

Updated README to inform OSX users that XCode doesn't need to be installed in order to get stsrted using `nvm`.

Fixes #921
Theodeus/nvm,edwmurph/nvm,edwmurph/nvm,Kuzirashi/nvm,dfackler/nvm,slariviere/nvm,ljharb/nvm,evocateur/nvm,ljharb/nvm,Kuzirashi/nvm,lukechilds/nvm,PeterDaveHello/nvm,creationix/nvm,PeterDaveHelloKitchen/nvm,Theodeus/nvm,slariviere/nvm,tlevine/nvm,lukechilds/nvm,tlevine/nvm,PeterDaveHello/nvm,evocateur/nvm,creationix/nvm,dfackler/nvm,PeterDaveHelloKitchen/nvm