include documentation on how to create the app and api endpoint

examples/dice_game/README.md

@@ -1,13 +1,72 @@
 # DiceGame
 
+The following example uses a basic [Phoenix](https://www.phoenixframework.org/)
+web application. For more elaborate examples, see [examples](/docs/instrumentation/erlang/examples/).
+
 To start your Phoenix server:
 
   * Run `mix setup` to install and setup dependencies
   * Start Phoenix endpoint with `mix phx.server` or inside IEx with `iex -S mix phx.server`
 
 Now you can visit [`localhost:4000`](http://localhost:4000) from your browser.
 
-For more information on the OpenTelemetry instrumentation and dice roll API in this example, check out the guide for getting started with [OpenTelemetry in Phoenix](https://opentelemetry.io/docs/instrumentation/erlang/getting-started/phoenix/)
+For more information on the OpenTelemetry instrumentation in this example, check out the guide for getting started with [OpenTelemetry](https://opentelemetry.io/docs/instrumentation/erlang/getting-started/)
+
+## How did we get here?
+
+To begin, use the `phx` generator to create a new project. To keep things simple we'll leave the database out for now with the `--no-ecto` flag.
+In a real app, you'll probably want to include ecto configured for your preferred database with the `--database` flag (`postgres` by default).
+
+```shell
+mix phx.new --no-ecto dice_game
+```
+
+### Rolling The Dice
+
+Now we'll add an API endpoint that will let us roll the dice and return a random
+number between 1 and 6.
+
+To start, we'll add a route to the `/api` scope in your router
+
+```elixir
+# lib/dice_game_web/router.ex
+scope "/api", DiceGameWeb do
+  pipe_through :api
+
+  get "/rolldice", DiceController, :roll
+end
+```
+
+Then we'll create a new file in the controllers folder for that module. We told
+the router that we will define a roll function, so we'll do that. It will return
+a `200` response code and the result of a `dice_roll` function, which we will
+emit a span for. We also want to set the value of the generated roll as an
+attribute on the span.
+
+```elixir
+# lib/dice_game_web/controllers/dice_controller.ex
+defmodule DiceGameWeb.DiceController do
+  use DiceGameWeb, :controller
+  require OpenTelemetry.Tracer, as: Tracer
+
+  def roll(conn, _params) do
+    send_resp(conn, 200, dice_roll())
+  end
+
+  defp dice_roll do
+    Tracer.with_span("dice_roll") do
+      roll = Enum.random(1..6)
+
+      Tracer.set_attribute(:roll, roll)
+
+      to_string(roll)
+    end
+  end
+end
+
+```
+
+If you point your browser/curl/etc. to [`localhost:4000/api/rolldice`](http://localhost:4000/api/rolldice) you should get a random number in response.
 
 Ready to run in production? Please [check our deployment guides](https://hexdocs.pm/phoenix/deployment.html).