MIT license badge added

Author: sujancse

README.md

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 ![Packagist Version](https://img.shields.io/packagist/v/sujan/php-csv-exporter)
 ![Packagist](https://img.shields.io/packagist/dt/sujan/php-csv-exporter?color=green)
+![GitHub](https://img.shields.io/github/license/sujancse/php-csv-exporter?color=yellow)
 
 ## Overview
 A fast and tiny PHP library to export data to CSV, Excel, etc. The library is based on a PHP generator. It used only 20MB memory to download 5 Million data, whereas without generator it will take up to 500MB of memory. Tested on Laravel 6.