chore: docs for custom queries

documentation/topics/custom-queries-and-mutations.md

@@ -0,0 +1,24 @@
+# Custom Queries & Mutations
+
+You can define your own queries and mutations in your schema,
+using Absinthe's tooling. See their docs for more.
+
+If you want to return resource types defined by AshGraphql, however,
+you will need to use `AshGraphql.load_fields/4` to ensure that any
+requested fields are loaded.
+
+For example:
+
+```elixir
+query do
+  field :custom_get_post, :post do
+    arg(:id, non_null(:id))
+
+    resolve(fn %{id: post_id}, resolution ->
+      with {:ok, post} when not is_nil(post) <- Ash.get(AshGraphql.Test.Post, post_id) do
+        AshGraphql.load_fields(post, AshGraphql.Test.Post, resolution)
+      end
+    end)
+  end
+end
+```

mix.exs

@@ -69,6 +69,7 @@ defmodule AshGraphql.MixProject do
         "documentation/topics/graphql-generation.md",
         "documentation/topics/modifying-the-resolution.md",
         "documentation/topics/relay.md",
+        "documentation/topics/custom-queries-and-mutations.md",
         "documentation/topics/upgrade.md",
         "documentation/dsls/DSL:-AshGraphql.Domain.md",
         "documentation/dsls/DSL:-AshGraphql.Resource.md",