Add a sample picture

Signed-off-by: Pekka Helenius <fincer89@hotmail.com>

README.md

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 # Java Bookstore Project
 
+![](sample_0.png)
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 - This repository contains school-related Java server-side project work for an imaginary book store with required/necessary back-end features & course requirement description.
 
 - This repository is linked to work of repository [java-server-programming - GitHub/Fincer](https://github.com/Fincer/java-server-programming).