doc: Buy me a coffee button to readme

README.md

@@ -89,6 +89,7 @@ We work long, hard, in our own free time at developing and improving Courseplay.
 
 [![paypal](https://www.paypalobjects.com/en_US/i/btn/btn_donateCC_LG.gif)](https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=7PDM2P6HQ5D56&item_name=Promote+the+development+of+Courseplay&currency_code=EUR&source=url)
 
+<a href="https://www.buymeacoffee.com/courseplay" target="_blank"><img src="https://cdn.buymeacoffee.com/buttons/v2/default-yellow.png" alt="Buy Me A Coffee" style="height: 60px !important;width: 217px !important;" ></a>
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 ## Contributors