Blog Post: Resource Creation Guidelines and Patterns

[PlagueHO]

The discussion within this PR: dsccommunity/ActiveDirectoryCSDsc#79 has led me to think about how we could help guide resource creators to create resources using patterns that adhere to DSC practices and principles.

It could cover:

1. When to use IsSingleInstance
2. When to break a resource into multiple smaller resources (e.g. avoiding the use of EmbeddedInstance) - Single Responsibility Principle for resources?
3. When to use Ensure.
4. How to define a resource where parts of the resource parameter content may be modified outside DSC (E.g. the ADUser, ADGroup pattern).
5. How to select parameters to use as a Key