docs: Provide more info on custom state attributes (#62)

* docs: Provide more info on custom state attributes

* Update documentation/tutorials/getting-started-with-ash-state-machine.md

Co-authored-by: Zach Daniel <zachary.s.daniel@gmail.com>

---------

Co-authored-by: Zach Daniel <zachary.s.daniel@gmail.com>

documentation/dsls/DSL:-AshStateMachine.md

@@ -24,7 +24,7 @@ Provides tools for defining and working with resource-backed state machines.
 | [`initial_states`](#state_machine-initial_states){: #state_machine-initial_states .spark-required} | `list(atom)` |  | The allowed starting states of this state machine. |
 | [`deprecated_states`](#state_machine-deprecated_states){: #state_machine-deprecated_states } | `list(atom)` | `[]` | A list of states that have been deprecated but are still valid. These will still be in the possible list of states, but `:*` will not include them. |
 | [`extra_states`](#state_machine-extra_states){: #state_machine-extra_states } | `list(atom)` | `[]` | A list of states that may be used by transitions to/from `:*`. See the docs on wildcards for more. |
-| [`state_attribute`](#state_machine-state_attribute){: #state_machine-state_attribute } | `atom` | `:state` | The attribute to store the state in. |
+| [`state_attribute`](#state_machine-state_attribute){: #state_machine-state_attribute } | `atom` | `:state` | The attribute to store the state in. You don't have to declare an attribute yourself, but it is possible. See the tutorial for more information. |
 | [`default_initial_state`](#state_machine-default_initial_state){: #state_machine-default_initial_state } | `atom` |  | The default initial state |
 
 

documentation/tutorials/getting-started-with-ash-state-machine.md

@@ -81,6 +81,47 @@ actions do
 end
 ```
 
+## Declaring a custom state attribute
+
+By default, a `:state` attribute is created on the resource that looks like this:
+
+```elixir
+attribute :state, :atom do
+  allow_nil? false
+  default AshStateMachine.Info.state_machine_initial_default_state(dsl_state)
+  public? true
+  constraints one_of: [
+    AshStateMachine.Info.state_machine_all_states(dsl_state)
+  ]
+end
+```
+
+You can change the name of this attribute, without declaring an attribute yourself, like so:
+
+```elixir
+state_machine do
+  initial_states([:pending])
+  default_initial_state(:pending)
+  state_attribute(:alternative_state) # <-- save state in an attribute named :alternative_state
+end
+```
+
+If you need more control, you can declare the attribute yourself on the resource:
+
+```elixir
+attributes do
+  attribute :alternative_state, :atom do
+    allow_nil? false
+    default :issued
+    public? true
+    constraints one_of: [:issued, :sold, :reserved, :retired]
+  end
+end
+```
+
+Be aware that the type of this attribute needs to be `:atom` or a type created with `Ash.Type.Enum`. Both the `default` and list of values need to be correct! 
+
+
 ## Making a resource into a state machine
 
 The concept of a state machine (in this case a "Finite State Machine"), essentially involves a single `state`, with specified transitions between states. For example, you might have an order state machine with states `[:pending, :on_its_way, :delivered]`. However, you can't go from `:pending` to `:delivered` (probably), and so you want to only allow certain transitions in certain circumstances, i.e `:pending -> :on_its_way -> :delivered`.