[Docs] add some OS X notes to the README

Update README.markdown

README.markdown

@@ -23,6 +23,11 @@ Note: We still have some problems with FreeBSD, because there is no pre-built bi
 Note: On OS X, if you do not have Xcode installed and you do not wish to download the ~4.3GB file, you can install the `Command Line Tools`. You can check out this blog post on how to just that:
  - [How to Install Command Line Tools in OS X Mavericks & Yosemite (Without Xcode)](http://osxdaily.com/2014/02/12/install-command-line-tools-mac-os-x/)
 
+Note: On OS X, if you have/had a "system" node installed and want to install modules globally, keep in mind that:
+ - When using nvm you do not need `sudo` to globally install a module with `npm -g`, so instead of doing `sudo npm install -g grunt`, do instead `npm install -g grunt`
+ - If you have an `~/.npmrc` file, make sure it does not contain any `prefix` settings (which is not compatible with nvm)
+ - You can (but should not?) keep your previous "system" node install, but nvm will only be available to your user account (the one used to install nvm). This might cause version mismatches, as other users will be using `/usr/local/lib/node_modules/*` VS your user account using `~/.nvm/versions/node/vX.X.X/lib/node_modules/*`
+
 Homebrew installation is not supported.
 
 ### Install script