Merge pull request #821 from esparta/master

[en] Adding Operating System Book

free-programming-books.md

@@ -425,6 +425,7 @@
 
 
 ####Operating systems
+* [How to Make a Computer Operating System (Work in Progress)](https://github.com/SamyPesse/How-to-Make-a-Computer-Operating-System)
 * [The Art of Unix Programming](http://catb.org/esr/writings/taoup/html/) - Eric S. Raymond
 * [The Little Book of Semaphores](http://greenteapress.com/semaphores/) - Allen B. Downey
 * [Operating Systems and Middleware](https://gustavus.edu/mcs/max/os-book/) (PDF and LaTeX)
@@ -854,7 +855,7 @@
 * [The Haskell Road to Logic, Math and Programming](http://fldit-www.cs.uni-dortmund.de/~peter/PS07/HR.pdf) (PDF)
 * [Wikibook Haskell](http://en.wikibooks.org/wiki/Haskell)
 * [Yet Another Haskell Tutorial](http://hal3.name/docs/daume02yaht.pdf) (PDF)
-* [Parallel and Concurrent Programming in Haskell](http://chimera.labs.oreilly.com/books/1230000000929/index.html) -  Simon Marlow 
+* [Parallel and Concurrent Programming in Haskell](http://chimera.labs.oreilly.com/books/1230000000929/index.html) -  Simon Marlow
 
 ###HTML / CSS
 * [A beginner's guide to HTML&CSS](http://learn.shayhowe.com/html-css/)