Auto merge of #5317 - konstin:patch-1, r=matklad
authorbors <bors@rust-lang.org>
Sat, 7 Apr 2018 18:58:39 +0000 (18:58 +0000)
committerbors <bors@rust-lang.org>
Sat, 7 Apr 2018 18:58:39 +0000 (18:58 +0000)
Add link to the sources of the book

It's quite hard to find the folder with the sources of the book, so I added the link to the introduction.


Trivial merge