Update README.md

Author: codingseb

README.md

@@ -67,3 +67,10 @@ The biggest difference of ExpressionEvaluator is that everything is evaluated on
 So it can be slower in some cases (sometimes not) but it also avoid a lot of memory leaks. 
 It already allow to evaluate some small scripts.
 If you don't want an another .dll file in your project, you only need to copy one [C# file](./CodingSeb.ExpressionEvaluator/ExpressionEvaluator.cs) in your project. And it's [MIT licence](./LICENSE.md)
+
+### Donate
+Expression Evaluator is free and will always be.
+But if you want to say thanks for this lib with a donation or small sponsoring here you can :  
+[Donate](https://www.paypal.com/donate?hosted_button_id=7K467U3H4NVJG)
+
+Thank you anyway for using ExpressionEvaluator