Add Article for PowerShell DSC

In reference to #6738, adding an article on using PowerShell DSC to install and configure Docker. This article compliments the existing Chef (https://docs.docker.com/articles/chef/) and Puppet (https://docs.docker.com/articles/puppet/) articles. The DSC configuration source referenced offers similar functionality to that of the Chef cookbook for Docker and Puppet module for Docker. This also aligns nicely with newly introduced Docker provisioning functionality in Azure as part of the Azure xplat-cli.

The community-contributed DSC source can be found at https://github.com/anweiss/DockerClientDSC.