Add paperback link to 'csintro' formats (#770)

docs/courses/csintro.md

@@ -11,7 +11,7 @@ This course takes approximately 14 weeks to complete, spending about 1 week on e
 ### ~ hint
 **Download it**
 
-The entire course is also available as a download. Choose any of these formats:
+The entire course is also available as a download or as a book. Choose any of these formats:
 
 |||
 |-|-|
@@ -19,6 +19,7 @@ The entire course is also available as a download. Choose any of these formats:
 | [![](/static/courses/csintro/icons/microsoft-onenote-24x24.png)](https://aka.ms/MakeCodeforMicroBitNotebook) | [- **OneNote** - Intro to CS with MakeCode for micro:bit](https://aka.ms/MakeCodeforMicroBitNotebook) |
 | [![](/static/courses/csintro/icons/adobe-pdf-file-icon-24x24.png)](https://1drv.ms/b/s!AqsgsTyHBmRBgQ1Fjzm5y5wKG75M) | [- **PDF** - Intro to CS with MakeCode for micro:bit](https://1drv.ms/b/s!AqsgsTyHBmRBgQ1Fjzm5y5wKG75M) |
 | [![](/static/courses/csintro/icons/apple-itunes-ibook-24x24.png)](https://itunes.apple.com/us/book/making-with-micro-bit/id1255260221?mt=11) | [- **iBooks** - Making with micro:bit](https://itunes.apple.com/us/book/making-with-micro-bit/id1255260221?mt=11) |
+| [![](/static/courses/csintro/icons/paperback-book-24x24.png)](http://www.lulu.com/shop/mary-douglas-kiang/making-with-makecode-microbit/paperback/product-23457217.html) | [- **Paperback** - Making with MakeCode and Micro:bit](http://www.lulu.com/shop/mary-douglas-kiang/making-with-makecode-microbit/paperback/product-23457217.html) |
 ### ~
 
 ### Lesson structure