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

README.markdown

@@ -17,6 +17,9 @@ Note: We still have some problems with FreeBSD, because there is no pre-built bi
  - [[#900] [Bug] nodejs on FreeBSD need to be patched ](https://github.com/creationix/nvm/issues/900)
  - [nodejs/node#3716](https://github.com/nodejs/node/issues/3716)  
 
+Note: On OSX, 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/)
+
 ### Install script
 
 To install or update nvm, you can use the [install script][2] using cURL: