Update free-programming-books.md

Add http://gitimmersion.com/

free-programming-books.md

@@ -491,6 +491,7 @@
 ###Git
 * [Pro Git](http://git-scm.com/book)
 * [Git From The Bottom Up](http://ftp.newartisans.com/pub/git.from.bottom.up.pdf) (PDF)
+* [Git Immersion] (http://gitimmersion.com)
 * [Git internals](https://github.com/pluralsight/git-internals-pdf/raw/master/drafts/peepcode-git.pdf) (PDF)
 * [Git Magic](http://www-cs-students.stanford.edu/~blynn/gitmagic/)
 * [Git Reference](http://www.gitref.org)