Add Article for PowerShell DSC

NOTE: This proposed article requires pre-requisite knowledge of configuration management tools. More specifically, knowledge of Windows PowerShell Desired State Configuration is required as this article would be relevant to Windows users. Similarly, those familiar with Chef and Puppet can relate to the corresponding articles on those tools.

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. Using PowerShell DSC, Windows users can install and configure Docker on targeted Ubuntu nodes. Usage examples have been included in the `dsc.md` article.