Create R Markdown doc template

[apreshill]

To do items:

• Update template content based on @dcossyleon's doc-demo site; see rendered

• Add png icon for project wizard to .dcf

• Cross-check with cookbook content for R Markdown documents

[apreshill]

@dcossyleon were you able to use the template from your PR #4 ? I suspect we can move the logo files to the skeleton folder (instead of resources), then set this in the template.yaml:

name: lesson_single
description: >
   A single R Markdown lesson
create_dir: TRUE

But I can't get it to work as is.