Create Learning Resources Markdown

[joseph-ProCogia]

• Create a core textbook md file
• Create a exam prep guide md file

[joseph-ProCogia]

@kieranjmartin @rajnibhaya @StanleyBrill

Learning resources file is here on dev: https://github.com/RConsortium/R-Certification-WG/blob/dev/Curriculum/Exam%20Readiness/Learning%20Resources.md

Please add any material you feel is relevant and we can organize later.