rewrote readme, usage and modifications rst

docs/source/modifications.rst

@@ -1,5 +1,63 @@
 How to modify the environment or write modules
 ===============================================
 
+Modifying levels
+----------------
+Varying levels are created by defining Walls, Floor or Doors in *.txt*-files (see [levels](marl_factory_grid/levels) for examples).
+Define which *level* to use in your *config file* as:
+
+>>> General:
+    level_name: rooms  # 'double', 'large', 'simple', ...
+
+... or create your own , maybe with the help of `asciiflow.com <https://asciiflow.com/#/>`_.
+Make sure to use `#` as `Walls`_ , `-` as free (walkable) floor, `D` for `Doors`_.
+Other Entities (define your own) may bring their own `Symbols`.
+
+.. _Walls: marl_factory_grid/environment/entity/wall.py
+.. _Doors: modules/doors/entities.py
 
 
+Modifying Entites
+----------------
+Entites are `Objects`_ that can additionally be assigned a position.
+Abstract Entities are provided.
+If you wish to introduce new entities to the enviroment just create a new module, ...
+
+.. _Objects: marl_factory_grid/environment/entity/object.py
+
+Modifying Groups
+----------------
+`Groups`_ are entity Sets that provide administrative access to all group members.
+All `Entities`_ are available at runtime as EnvState property.
+
+.. _Groups: marl_factory_grid/environment/groups/objects.py
+.. _Entities: marl_factory_grid/environment/entity/global_entities.py
+
+Modifying Rules
+----------------
+`Rules`_ define how the environment behaves on microscale.
+Each of the hookes (`on_init`, `pre_step`, `on_step`, '`post_step`', `on_done`)
+provide env-access to implement customn logic, calculate rewards, or gather information.
+If you wish to introduce new rules to the environment....
+
+.. _Rules: marl_factory_grid/environment/entity/object.py
+
+.. image:: ./images/Hooks_FIKS.png
+   :alt: Hooks Image
+
+Modifying Results
+----------------
+`Results`_ provide a way to return `rule` evaluations such as rewards and state reports
+back to the environment.
+
+.. _Results: marl_factory_grid/utils/results.py
+
+Modifying Assets
+----------------
+Make sure to bring your own assets for each Entity living in the Gridworld as the `Renderer` relies on it.
+PNG-files (transparent background) of square aspect-ratio should do the job, in general.
+
+.. image:: ./marl_factory_grid/environment/assets/wall.png
+   :alt: Wall Image
+.. image:: ./marl_factory_grid/environment/assets/agent/agent.png
+   :alt: Agent Image