fix: Missing links in README.md

Author: Daandeve

README.md

@@ -67,5 +67,5 @@ This project is licensed under the MIT License. See the [LICENSE](LICENSE) file
 
 ## Acknowledgments
 
-1. OpenProvider for their robust API.
-1. FOSSBilling for their open-source billing platform.
+1. [OpenProvider](https://www.openprovider.com/) for their robust API.
+1. [FOSSBilling](https://fossbilling.org/) for their open-source billing platform.