Added a suggestion to star the GitHub repo

Author: egonw

index.i.md

@@ -41,7 +41,8 @@ License: CC-BY-SA 4.0 International
 This book is being opensourced. This involves transforming the LaTeX source into Markdown,
 and updating all scripts to ensure all the automation works well. I have made good
 steps forward, but it will take some time for things to iron out. If you find issue,
--please report them [here](https://github.com/egonw/cdkbook/issues).
+please report them [here](https://github.com/egonw/cdkbook/issues). If you like this
+book, please give the [GitHub repository](https://github.com/egonw/cdkbook/) a star.
 
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