Added ECS docs

docs/entity/entity.md

@@ -0,0 +1,37 @@
+# Entity
+### Contents
+[About](#About)
+
+[Usage](#Usage)
+
+[Notes](#Notes)
+
+## About
+An 'entity' is a unique ID.
+
+For most implementations, it makes sense to use a simple integer value - usually in a packed array (*reuses the most minimal value it can*)
+
+E.g.: The ID 101 would be the next selected value for a new entity.
+
+>100 | 101 | 102| 103| 104| ...|
+>:----:|:-----:|:----:|:----:|:----:|:----|
+><span style="color:gray">used|unused|<span style="color:gray">used|<span style="color:gray">used| unused| <span style="color:gray">....
+<br>
+
+**This ID represents a specific instance of a game object**
+
+e.g. character, vehicle, effect etc.
+
+___An entity contains no further data.___
+
+In order to be useful, its identifier must be registered to one or more [components](./component/component.md).
+
+## Usage
+*some specific usage info here*
+``` java
+void todo();
+```
+
+## Notes
+* The only requirement for a Entity is that it has a unique ID. An [entity manager](./manager/entity_manager.md) could implement a more complex ID system such as using a packed array for only a small region of IDs, or using named IDs by use of a dictionary or ordered hash-map
+* The maximum number of entities is limited by the size of the container used to store it. E.g., for a 32-bit unsigned integer, there are ~4.3 billion ID's available. But keep in mind that such a large amount of IDs will take significant time for each system to consider.