Add material for information (not part of the program)

[lgatto]

Clarify that some material is for information (for example ggplot fine-tuning)

From Toby:

you can use inline Instructor Notes for blocks that will only be visible in Instructor View

::::::: instructor
Instructor note goes here
:::::: 

or 'spoiler' class blocks for expandable/collapsible blocks

::::::::: spoiler
expand me!
collapse me!
::::::::