doc: add fennecs to similar projects

Author: skypjack

docs/md/reference.md

@@ -50,6 +50,8 @@ details.
     inspired archetype ECS with optional multithreading. 
   * [Entitas](https://github.com/sschmid/Entitas-CSharp): the ECS framework for
     C# and Unity, where _reactive systems_ were invented.
+  * [Fennecs](#https://github.com/outfox/fennecs): the little archetype ECS that
+    loves you back.
   * [Friflo ECS](https://github.com/friflo/Friflo.Engine.ECS): an archetype ECS
     with focus on performance and minimal GC allocations.
   * [LeoECS](https://github.com/Leopotam/ecs): simple lightweight C# Entity