Add reference to npc.py example from NPC tutorial

docs/source/Contribs/Contrib-Containers.md

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+# Containers
+Contribution by InspectorCaracal (2023)
+
+Adds the ability to put objects into other container objects by providing a container typeclass and extending certain base commands.
+
+## Installation
+
+To install, import and add the `ContainerCmdSet` to `CharacterCmdSet` in your `default_cmdsets.py` file:
+
+```python
+from evennia.contrib.game_systems.containers import ContainerCmdSet
+
+class CharacterCmdSet(default_cmds.CharacterCmdSet):
+    # ...
+    
+    def at_cmdset_creation(self):
+        # ...
+        self.add(ContainerCmdSet)
+```
+
+This will replace the default `look` and `get` commands with the container-friendly versions provided by the contrib as well as add a new `put` command.
+
+## Usage
+
+The contrib includes a `ContribContainer` typeclass which has all of the set-up necessary to be used as a container. To use, all you need to do is create an object in-game with that typeclass - it will automatically inherit anything you implemented in your base Object typeclass as well.
+
+    create bag:game_systems.containers.ContribContainer
+
+The contrib's `ContribContainer` comes with a capacity limit of a maximum number of items it can hold. This can be changed per individual object.
+
+In code:
+```py
+obj.capacity = 5
+```
+In game:
+
+    set box/capacity = 5
+
+You can also make any other objects usable as containers by setting the `get_from` lock type on it.
+
+    lock mysterious box = get_from:true()
+
+## Extending
+
+The `ContribContainer` class is intended to be usable as-is, but you can also inherit from it for your own container classes to extend its functionality. Aside from having the container lock pre-set on object creation, it comes with three main additions:
+
+### `capacity` property
+
+`ContribContainer.capacity` is an `AttributeProperty` - meaning you can access it in code with `obj.capacity` and also set it in game with `set obj/capacity = 5` - which represents the capacity of the container as an integer. You can override this with a more complex representation of capacity on your own container classes.
+
+### `at_pre_get_from` and `at_pre_put_in` methods
+
+These two methods on `ContribContainer` are called as extra checks when attempting to either get an object from, or put an object in, a container. The contrib's `ContribContainer.at_pre_get_from` doesn't do any additional validation by default, while `ContribContainer.at_pre_put_in` does a simple capacity check.
+
+You can override these methods on your own child class to do any additional capacity or access checks.
+
+----
+
+<small>This document page is generated from `evennia/contrib/game_systems/containers/README.md`. Changes to this
+file will be overwritten, so edit that file rather than this one.</small>