Add usage-rules.md for AI assistant context

Author: jechol

usage-rules.md

@@ -0,0 +1,75 @@
+# Rules for working with AshRandomParams
+
+AshRandomParams generates random parameters for Ash resource actions. Useful for test data generation.
+
+## Setup
+
+```elixir
+defmodule MyApp.Post do
+  use Ash.Resource, extensions: [AshRandomParams]
+
+  random_params do
+    random MyRandom  # Optional: custom generator
+  end
+end
+```
+
+## Basic Usage
+
+```elixir
+# Generate random params
+Post.random_params!(:create)
+
+# With initial params
+Post.random_params!(:create, %{author: "James"})
+
+# With options
+Post.random_params!(:create, %{}, %{
+  enforce_random: [:content],    # Force random generation
+  exclude: [:title],             # Skip generation
+  include_defaults?: false       # Exclude defaults
+})
+```
+
+## Default Behavior
+
+Generates random values only for accepts/arguments with `allow_nil?: false` and `default == nil`.
+
+For `belongs_to` relationship foreign keys:
+- `create` actions: generated as `nil`
+- Other actions: not generated (preserves existing relationship)
+
+## Custom Random Generator
+
+```elixir
+defmodule MyRandom do
+  use AshRandomParams.Random
+
+  @impl true
+  def random(%{type: Ash.Type.Integer}, _opts, _context) do
+    777
+  end
+
+  @impl true
+  def random(attr_or_arg, opts, context) do
+    # Fallback to DefaultRandom for other types
+    AshRandomParams.DefaultRandom.random(attr_or_arg, opts, context)
+  end
+end
+```
+
+## Options
+
+- **enforce_random**: Force random generation for specific fields
+- **exclude**: Skip generation for specific fields
+- **include_defaults?**: Include fields with `allow_nil?: true` or non-nil defaults (default: `true`)
+
+## Testing
+
+```elixir
+test "create post" do
+  params = Post.random_params!(:create, %{author: "Test Author"})
+  assert {:ok, post} = MyApp.Blog.create_post(params)
+end
+```
+