updated Prolog link, added PDF version (fixes #868) (#873)

* updated Prolog link, added PDF version

* updated Prolog link, added PDF version

- removed `Text` column, added footnote instead

* updated Prolog link, added PDF version

- parentheses

* updated Prolog link, added PDF version

- added link to book source code

README.md

@@ -344,15 +344,17 @@ If not, or if a student chooses not to take the Capstone, then a separate Final
 `and more`
 
 Courses | Duration | Effort | Prerequisites
-:-- | :--: | :--: | :--:
+:-- | :--: | :--: | :--: 
 [Parallel Programming](https://www.coursera.org/learn/parprog1)| 4 weeks | 6-8 hours/week | Scala programming
 [Compilers](https://www.edx.org/course/compilers) | 9 weeks | 6-8 hours/week | none
 [Introduction to Haskell](https://www.seas.upenn.edu/~cis194/fall16/)| 14 weeks | - | -
-[Learn Prolog Now!](http://www.learnprolognow.org/lpnpage.php?pageid=online)| 12 weeks | - | -
+[Learn Prolog Now!](https://www.let.rug.nl/bos/lpn//lpnpage.php?pageid=online) ([alt](https://github.com/ossu/computer-science/files/6085884/lpn.pdf))*| 12 weeks | - | -
 [Software Debugging](https://www.udacity.com/course/software-debugging--cs259)| 8 weeks | 6 hours/week | Python, object-oriented programming
 [Software Testing](https://www.udacity.com/course/software-testing--cs258) | 4 weeks | 6 hours/week | Python, programming experience
 [Software Architecture & Design](https://www.udacity.com/course/software-architecture-design--ud821)| 8 weeks | 6 hours/week | software engineering in Java
 
+(*) book by Blackburn, Bos, Striegnitz (compiled from [source](https://github.com/LearnPrologNow/lpn), redistributed under [CC license](http://creativecommons.org/licenses/by-sa/4.0/))
+
 ### Advanced systems
 
 **Topics covered**: