Update README.markdown
Update README.markdown

Added notes for OS X users regarding globally installed modules.

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