Add link to CS107 unix videos in readings

commit 692c87eb8d3bd78ec705b81c2fba74ff6a392259
Author: Julie Zelenski <[email protected]>
Date:   Mon Jan 7 17:42:37 2019 -0800

    Add link to CS107 unix videos in readings

Author: zelenski

lectures/Intro/README.md

@@ -4,5 +4,6 @@ permalink: /lectures/Intro/
 title: Introduction and Welcome
 readings: |
     + Before friday's lecture, review our guides on [electricity](/guides/electricity/), [binary/hexadecimal numbers](/guides/numbers/), and the [unix command line](/guides/unix). 
+    + Peruse this collection of [video tutorials](https://web.stanford.edu/class/archive/cs/cs107/cs107.1186/unixref/) that were prepared to introduce unix to CS107 students. These videos cover basic concepts, common utilities, development tools, and some fancy productivity features -- something for everyone!
     + Check out this fun read on [how the Raspberry Pi came to be](https://www.techrepublic.com/article/inside-the-raspberry-pi-the-story-of-the-35-computer-that-changed-the-world/) for a taste of the adventure to come!
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