Carol/docs/doc 803

Create wiki documentation: topic for course authors and shared topic for students.
@mhoeber @lamagnifica @srpearce @JAAkana @explorerleslie please review. 
"You can enter plain text and use Markdown syntax to add formatting" implies that the user has a choice (for example, she could use something other than plain text). Maybe make this a little tighter?

"cheat sheet"

Line wrap