Add local rendering info

This adds information on how to preview content, both locally and online.

I wasn't sure where to put the information, so this is still marked Draft. Right now I have it in its own file, so its content can be reviewed. This file can be previewed here.

In the longer run, I think the most logical thing might be to put this information in the file which @mlangseth created in !2 (merged) , once that is merged. That way, all of our course-developer-specific stuff will be in one place.

I'm assigning @csherwood as the reviewer here, since he had a desire to do local preview.