Update README.md

Author: miccol

README.md

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-<img width="200" align="left" src="http://michelecolledanchise.com/wp-content/uploads/2018/05/btbook.jpg">
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+<a href="https://btirai.github.io"><img width="200" align="left" url= "https://btirai.github.io/" src="http://michelecolledanchise.com/wp-content/uploads/2018/05/btbook.jpg"></a>
 Our book **Behavior Trees in Robotics and AI**, published by CRC Press Taylor & Francis, is available for purchase (ebook and hardcover) on the CRC Press Store or Amazon. The Preprint version (**free**) is available here: https://arxiv.org/abs/1709.00084<br><br>
 **Tutorials** available at https://btirai.github.io/