Added a little bit about "directory environments"

Starting in puppet 3.5, dynamic environments could be configured using some new configuration directives. Apparently the modulepath directive will be deprecated in puppet 4. I've added a bit in the introduction page to explain this to newcomers.
I would like to keep this section mainly focused on the concept and not talk about the exact configuration details and upcoming deprecations. Could we move the precise details into https://github.com/adrienthebo/r10k/blob/master/doc/dynamic-environments/master-configuration.mkd and just have this document link to the master-configuration dock?