Add some Instructor Notes

[tobyhodges]

At the moment, the Instructor Notes page contains content relevant to the original version of the lesson. This should be replaced with guidance relevant to the redesigned lesson.

Based on feedback so far, some points we could add in:

• Some explanation of the rationale behind placing ggplot first
• Guidance on how to prepare learners for the differences between tidyverse's way of doing things and the normal operations/syntax of base R