Pre-release of Template

This is a first "release". There are a few more issues to be addressed before we call this version 1.0 but this allows us to check the releasing system.

Template/style changes:

• References.Rmd is a new file where all references are printed out. (Note it is added to the _bookdown.yml file).
• About.Rmd contains a template credit table that guides you through how to write credits.

Github action changes:

• render-preview.yml This Github action will create a preview branch upon opening a pull request and will update a comment on the PR that will bring you to a preview of what your changes make the Bookdown and Coursera versions of the course will look like. (Note that this does not include any Docker changes)
• delete-preview.yml deletes the preview branch created by render-preview.yml after the pull request is closed.
• transfer-rendered-files.yml will not transfer the docs/coursera folder files to the `_Leanpub repository.

Docker image updates:

• Bookdown has been updated 0.20 to 0.24
• vim has been added.