add $5 campaign

Author: Packt-ITService

README.md

@@ -1,3 +1,8 @@
+## $5 Tech Unlocked 2021!
+[Buy and download this Book for only $5 on PacktPub.com](https://www.packtpub.com/product/android-system-programming/9781787125360)
+-----
+*If you have read this book, please leave a review on [Amazon.com](https://www.amazon.com/gp/product/178712536X).     Potential readers can then use your unbiased opinion to help them make purchase decisions. Thank you. The $5 campaign         runs from __December 15th 2020__ to __January 13th 2021.__*
+
 # Android System Programming
 
 This is the code repository for [Android System Programming](https://www.packtpub.com/application-development/android-system-programming?utm_source=GitHub&utm_medium=repo&utm_campaign=9781787125360), published by [Packt](www.packtpub.com). It contains all the supporting project files necessary to work through the book from start to finish.