docs: add doc for use-subscriptions-with-graphql.md

documentation/how_to/use-subscriptions-with-graphql.md

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+# Using Subscriptions
+
+The AshGraphql DSL does not currently support subscriptions. However, you can do this with Absinthe direclty, and use `AshGraphql.Subscription.query_for_subscription/3`. Here is an example of how you could do this for a subscription for a single record. This example could be extended to support lists of records as well.
+
+```elixir
+# in your absinthe schema file
+subscription do
+  field :field, :type_name do
+    config(fn
+      _args, %{context: %{current_user: %{id: user_id}}} ->
+        {:ok, topic: user_id, context_id: "user/#{user_id}"}
+
+      _args, _context ->
+        {:error, :unauthorized}
+    end)
+
+    resolve(fn args, _, resolution ->
+      # loads all the data you need
+      AshGraphql.Subscription.query_for_subscription(
+        YourResource,
+        YourAPi,
+        resolution
+      )
+      |> Ash.Query.filter(id == ^args.id)
+      |> YourAPi.read(actor: resolution.context.current_user)
+    end)
+  end
+end
+```