RFC: Migration to R 4.0

[zkamvar]

During the June Maintainer meeting, we asked for comments and experiences with the migration to R 4.0 so that we could create guidance for maintainers and instructors. We have drafted a short blog post (https://github.com/carpentries/carpentries.org/pull/830) to be released next week (2020-08-03) that describes our recommendations for migration. You can find a preview of the blog post here. Please look over the blog posts and make comments by 2020-07-30 so that we can incoroporate any changes before the post goes live.

To help identify the differences between R 3.6 and R 4.0, I have run this lesson in both versions and posted the results that show the differences in output chunks and entire markdown files.