add links to ISLR videos - closes #58

Linking to my blog post instead of individual YouTube videos because:

* The YouTube videos have worthless titles, whereas my post has descriptive video titles
* My post links to the slide decks, so students can browse the video contents to decide whether they want to watch them
* The YouTube videos are unlisted (so you can't browse to see what others are available), whereas on my post you can discover other videos you might like to watch