Add SolidInvoice to Money, Budgeting & Management (#3778)

Co-authored-by: nodiscc <nodiscc@gmail.com>

README.md

@@ -1409,6 +1409,7 @@ _Related: [Inventory Management](#inventory-management), [Resource Planning - En
 - [OpenBudgeteer](https://github.com/TheAxelander/OpenBudgeteer) - A budgeting app based on the Bucket Budgeting Principle. `MIT` `Docker/C#`
 - [REI3](https://rei3.de/home_en/) - Open source, expandable Business Management Software. Manage tasks, time, assets and much more. ([Demo](https://rei3.de/demo_en/), [Source Code](https://github.com/r3-team/r3)) `MIT` `Go`
 - [SilverStrike](https://silverstrike.org/) - Personal finance management made easy. ([Demo](https://demo.silverstrike.org/), [Source Code](https://github.com/agstrike/silverstrike)) `MIT` `Python/Django`
+- [SolidInvoice](https://solidinvoice.co) - Open source invoicing and quote application. ([Source Code](https://github.com/SolidInvoice/SolidInvoice)) `MIT` `PHP`
 - [StockazNG](https://dev.sigpipe.me/dashie/StockazNG) - Asset Management System. `MIT` `Python`