Ahodges/documentation/doc764

@WatsonEmily , please distribute for PM review for usability by our audience.
@jhelbert , @stroilova , @shnayder , @rocha  please review for accuracy, completeness, and level of detail, particularly wrt data czars who will need to revise scripts for post-download processing.
@mhoeber , @srpearce please copy edit.
I might mention here that this file is planned for removal in the near future and that existing users should transition any existing infrastructure that relies on this file to use the new file found in edx-course-data.

Also not sure if you want to call out that this file is found in course-data instead of edx-course-data here.