Add a directory to your Mac OSX web sharing server by editing apache conf

dzello.conf

#substitute 'dzello' in these examples with your OSX account's username
#substitute the illustrative repository path (preview_markdown_locally) with your repository path

#add these lines to the file at /etc/apache2/users/dzello.conf

Alias /~dzello/preview_markdown_locally "/Users/dzello/workspace/preview_markdown_locally"
<Directory "/Users/dzello/workspace/preview_markdown_locally">
  Order allow,deny
  Allow from all
</Directory>

#then uncheck and recheck 'Web Sharing' in OSX > Settings > Sharing
#you're done! now navigate your browser to:

#http://localhost/~dzello/preview_markdown_locally/README.html

#check out http://blog.joshdzielak.com/view-and-iterate-on-your-github-readmemd-loca for an explanation of preview_markdown_locally