Update README.md: Add Mailgen to the list of projects using Fluid

README.md

@@ -1135,5 +1135,6 @@ Fluid is known to be used in the following projects:
 - [TemplateTo](https://templateto.com) Powerful Template Based Document Generation
 - [Weavo Liquid Loom](https://www.weavo.dev) A Liquid Template generator/editor + corresponding Azure Logic Apps Connector / Microsoft Power Automate Connector
 - [Semantic Kernel](https://github.com/microsoft/semantic-kernel) Integrate cutting-edge LLM technology quickly and easily into your apps
+- [Mailgen](https://github.com/hsndmr/Mailgen) A .NET package that generates clean, responsive HTML e-mails for sending transactional mail.
 
 _Please create a pull-request to be listed here._