added books and courses to r-resources.md

[orthodoc]

specifics: added data science specialization course from coursera, machine learning
course from coursera and a book on machine learning for hackers.

[orthodoc]

I see that there have been pull requests previously on the resources I suggested for R. And discussions are one regarding including or removing them. So I am closing this PR.

[clarecorthell]

Thank you for submitting a contribution! As I describe in greater detail here, I'm very exclusive with the material in the core curriculum. If you'd make a strong case for any of the resources you PR, please do so.

It is true that I don't believe ML for Hackers is a very valuable resource. It doesn't focus on python, which is the primary reason for leaving it out. As for the specialization, it is useful for data analysis with R, but I don't believe it's valuable enough to note where the curriculum is focused on python.

Thank you! Contributions make this curriculum what it is. I appreciate PRs regardless of whether I can incorporate them.