adds link to paperback version

manuscript/00.0-preface.Rmd

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 The book focuses on machine learning models for tabular data (also called relational or structured data) and less on computer vision and natural language processing tasks.
 Reading the book is recommended for machine learning practitioners, data scientists, statisticians, and anyone else interested in making machine learning models interpretable.
 
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 `r if(is.html){"You can buy the PDF and e-book version (epub, mobi) [on leanpub.com](https://leanpub.com/interpretable-machine-learning)."}`
 
+`r if(is.html){"You can buy the print version [on lulu.com](http://www.lulu.com/shop/christoph-molnar/interpretable-machine-learning/paperback/product-24036234.html)."}`
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 **About me:** My name is Christoph Molnar, I'm a statistician and a machine learner.
 My goal is to make machine learning interpretable.
 If you are interested in improving the interpretability of your machine learning models, do not hesitate to contact me!