Revisit the Resources for R section

[laijasmine]

Provide links additional material that might be helpful for interns who are hoping to improve their R skills beyond the initial training.

• make the Resources for R it's own section
• highlight specific chapters that might be relevant in books eg. Chapter 3 on programming in Efficient R
• add ADC training materials http://training.arcticdata.io/materials/

[laijasmine]

may be updated by the hackyhour group, if so we can potentially just link to the webpage