add free ebook notification

README.md

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+### Get this product for $5
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+<i>Packt is having its biggest sale of the year. Get this eBook or any other book, video, or course that you like just for $5 each</i>
+
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+<b><p align='center'>[Buy now](https://packt.link/9781838824419)</p></b>
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+<b><p align='center'>[Buy similar titles for just $5](https://subscription.packtpub.com/search)</p></b>
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+
 # Hands-On Music Generation with Magenta
 
 [![Magenta Version 1.1.7](./docs/magenta-v1.1.7-badge.svg)](https://github.com/magenta/magenta/releases/tag/1.1.7)