Added book to Machine Learning section

Textbook draft of Probabilistic Models in the Study of Language.

free-programming-books.md

@@ -260,6 +260,7 @@
 * [Gaussian Processes for Machine Learning](http://www.gaussianprocess.org/gpml/)
 * [Information Theory, Inference, and Learning Algorithms](http://www.inference.phy.cam.ac.uk/itila/)
 * [Artificial Intelligence | Machine Learning](http://see.stanford.edu/see/materials/aimlcs229/handouts.aspx) - Andrew Ng *(Notes, lectures, and problems)*
+* [Probabilistic Models in the Study of Language](http://idiom.ucsd.edu/~rlevy/pmsl_textbook/text.html) (Draft, with R code)
 
 ####Mathematics
 * [Think Bayes: Bayesian Statistics Made Simple](http://www.greenteapress.com/thinkbayes/) - Allen B. Downey