Update README.md

Author: miccol

README.md

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 NEWS!
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-The Preprint of our book titled "Behavior Trees in Robotics and AI" is available here: https://arxiv.org/abs/1709.00084
+The Preprint of our book titled "**Behavior Trees in Robotics and AI**" is available here: https://arxiv.org/abs/1709.00084
 
 <img width="100" alt="portfolio_view" src="http://michelecolledanchise.com/BTPPLOGO.png"> BT++ ![License MIT](https://img.shields.io/dub/l/vibe-d.svg)
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