chore(README.md): preview image

Author: FrancescoBorzi

README.md

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 [![Build Status](https://travis-ci.com/azerothcore/Keira3.svg?branch=master)](https://travis-ci.com/azerothcore/Keira3)
 <a href="https://www.paypal.me/francesco92dev" target="_blank"><img src="https://img.shields.io/badge/Donate-PayPal-ff3f59.svg"/></a>
 
+![Keira3](https://user-images.githubusercontent.com/75517/59927799-8eba4a00-943d-11e9-876e-06f4b08e01a2.png)
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 Cross-platform desktop application featuring a **Database Editor** for the [AzerothCore MMORPG framework](http://www.azerothcore.org).
 
 With Keira3 you don't have to know the SQL language in order to change contents: it will generate the SQL queries automatically for you. The SQL code will be displayed, so you can **learn**, and then you can **copy** or **execute** it directly to your database.