Add a DSC Authoring document detailing design pattern for Single Instance resources

[PlagueHO]

At the moment there is no recommended design pattern for authoring DSC Resources where only a single instance can be used on a node. A method has been documented (here)[http://blogs.msdn.com/b/powershell/archive/2015/06/23/want-to-write-a-dsc-resource-where-only-a-single-instance-can-be-configured.aspx]

Could I suggest a new document is created /dsc/AuthoringResourceSingleInstance.md that contains the information @TravisEz13 provided in his blog article?

[ghost]

@PlagueHO @TravisEz13, I have a new topic in my working branch on this:
https://github.com/PowerShell/PowerShell-Docs/blob/esles-working/dsc/singleInstance.md

Would you be willing to take a look at the topic and let me know what you think before I publish it live?

I didn't include the class-based example, as I thought it was out-of-scope for this particular topic, but I'm considering using xTimeZone as a script vs. class resource example for another topic.

Thanks!!

[PlagueHO]

@eslesar:
Looking pretty good to me - couple of things:

• Resource typo (spelled "Resoource").
• It is probably fairly obvious, but could we add some detail as to why/what happens when a resource is not unique? E.g. (addition in bold).
  "Because of this, a configuration such as the one above would compile and run without warning. This would cause the configuration to be repeatedly applied to the node, cycling the timezone back and forth."
• Some of the { and } need to be on new lines in the "Updated script:" so that it matches the DSC resource style guidelines (although this really is me being a bit too nit picky and is because the original DSC Resource doesn't meet them 😄 ). I'll submit a PR to update the xTimezone resource.

Other than those minor points the doc look great to me.

[ghost]

Thanks for the input, @PlagueHO! Changes made and issue addressed with #271

[PlagueHO]

@eslesar - nice! Thank you very much 👍 I've submitted a PR to the xTimezone repo to ensure the code adheres to the styleguidelines. Once accepted I'll pop a PR through to update the code samples in this document. Thanks again.