added OpenGTS to Maps & GPS section (#662)

* added OpenGTS to Maps & GPS section

OpenGTS™ ("Open GPS Tracking System") is the first available open source project designed specifically to provide web-based GPS tracking services for a "fleet" of vehicles

* updates according to pull request comments

README.md

@@ -588,6 +588,7 @@ Some [Groupware](#groupware) solutions also feature file sharing and synchroniza
 
   * [Graphhopper](https://graphhopper.com/) - Fast routing library and server using OpenStreetMap. ([Source Code](https://github.com/graphhopper/graphhopper))
   * [MapBBCodeShare](https://github.com/MapBBCode/share.mapbbcode.org) - Tool for sharing custom OSM maps. Support for annotated markers, polygons, lines, multi-format import/export, multiple layers, shortlinks... ([Demo](http://share.mapbbcode.org/)) `WTFPL/Other` `PHP/Javascript`
+  * [OpenGTS](http://www.opengts.org/) - Entry-level fleet tracking system. Supports variety of tracking devices and protocols. Comes with rich web-interface and reporting features. ([Demo](http://track.opengts.org/track/Track), [Source Code](https://sourceforge.net/projects/opengts/files/server-base/)) `Apache` `Java`
   * [Openstreetmap](http://www.openstreetmap.org/) - OpenStreetMap is a map of the world, created by people like you and free to use under an open license. ([Source Code](https://github.com/openstreetmap/openstreetmap-website))
   * [Traccar](https://www.traccar.org/) - Java application to track GPS positions. Supports loads of tracking devices and protocols, has an Android and iOS App. Has a web interface to view your trips. ([Demo](http://demo.traccar.org/), [Source Code](https://github.com/tananaev?tab=repositories)) `Apache`
   * [uMap](https://umap.openstreetmap.fr/en/) - Create maps with OpenStreetMap layers in a minute and embed them in your site. ([Source Code](https://github.com/umap-project/umap)) `WTFPL` `Python/Django`