Added Firefly III to financial tools (#618)

* Added Firefly III to financial tools

* Added correct license (MIT) for Firefly III

* Shortened description as for contribution guidelines

README.md

@@ -635,6 +635,7 @@ See also <https://en.wikipedia.org/wiki/List_of_streaming_media_systems>, <https
 
   * [Benedetto](https://github.com/arecker/bennedetto) - Bennedetto is a simple, turn-based budget management app. `GPLv3` `Python`
   * [Economizzer](http://www.economizzer.org/) - Economizzer is a simple and open source personal finance manager system, available in English, Brazilian, Portuguese and Russian. ([Source Code](https://github.com/gugoan/economizzer)) `MIT` `PHP`
+  * [Firefly III](https://jc5.github.io/firefly-iii/) - Firefly III is a financial manager. It helps you keeping track of your money and make budget forecasts. It supports credit cards, shared household accounts and savings accounts. Very modern, it's powered by Laravel and requires PHP7. `MIT` `PHP`
   * [Galette](http://galette.eu/dc/) - Galette is a membership management web application towards non profit organizations. ([Source Code](http://git.tuxfamily.org/galette/galette.git/)) `GPLv3` `PHP`
   * [GLPI](http://www.glpi-project.org/spip.php?lang=en) - GLPI is an IT assests/resources/inventory manager. ([Source Code](https://forge.glpi-project.org/projects/glpi)) `GPLv2` `PHP`
   * [GRR](http://grr.devome.com/?lang=en) - Assets management and booking for small/medium companies. ([demo](http://grr.devome.com/demo/week_all.php?year=2015&month=07&day=28&area=94), _administrateur/azerty_) `GPLv2` `PHP`