docs: Add GitHub quick start guide. (#143)

documentation/tutorials/github-quickstart.md

@@ -0,0 +1,86 @@
+# GitHub Quick Start Guide
+
+This is a _very quick_ tutorial on how to configure your application to use
+GitHub for authentication.
+
+First you need to configure an application in your [GitHub developer
+settings](https://github.com/settings/developers):
+
+  1. Click the "New OAuth App" button.
+  2. Set your application name to something that identifies it.  You will likely
+     need separate applications for development and production environments, so
+     keep that in mind.
+  3. Set "Homepage URL" appropriately for your application and environment.
+  4. In the "Authorization callback URL" section,  add your callback URL.  The
+     callback URL is generated from the following information:
+      - The base URL of the application - in development that would be
+        `http://localhost:4000/` but in production will be your application's
+        URL.
+      - The mount point of the auth routes in your router - we'll assume
+        `/auth`.
+      - The "subject name" of the resource being authenticated - we'll assume `user`.
+      - The name of the strategy in your configuration.  By default this is
+        `github`.
+
+     This means that the callback URL should look something like
+     `http://localhost:4000/auth/user/github/callback`.
+  5. Do not set "Enable Device Flow" unless you know why you want this.
+  6. Click "Register application".
+  7. Click "Generate a new client secret".
+  8. Copy the "Client ID" and "Client secret" somewhere safe, we'll need them
+     soon.
+  9. Click "Update application".
+
+Next we can configure our resource (assuming you already have everything else
+set up):
+
+```elixir
+defmodule MyApp.Accounts.User do
+  use Ash.Resource, extensions: [AshAuthentication]
+
+  authentication do
+    github do
+      client_id MyApp.Secrets
+      redirect_uri MyApp.Secrets
+      client_secret MyApp.Secrets
+    end
+  end
+end
+```
+
+Because all the configuration values should be kept secret (ie the
+`client_secret`) or are likely to be different for each environment we use the
+`AshAuthentication.Secret` behaviour to provide them. In this case we're
+delegating to the OTP application environment, however you may want to use a
+system environment variable or some other secret store (eg Vault).
+
+```elixir
+defmodule MyApp.Secrets do
+  use AshAuthentication.Secret
+
+  def secret_for([:authentication, :strategies, :github, :client_id], MyApp.Accounts.User, _) do
+    get_config(:client_id)
+  end
+
+  def secret_for([:authentication, :strategies, :github, :redirect_uri], MyApp.Accounts.User, _) do
+    get_config(:redirect_uri)
+  end
+
+  def secret_for([:authentication, :strategies, :github, :client_secret], MyApp.Accounts.User, _) do
+    get_config(:client_secret)
+  end
+
+  defp get_config(key) do
+    :my_app
+    |> Application.get_env(:github, [])
+    |> Keyword.fetch(key)
+  end
+end
+```
+
+The values for this configuration should be:
+
+  * `client_id` - the client ID copied from the GitHub settings page.
+  * `redirect_uri` - the URL to the generated auth routes in your application
+    (eg `http://localhost:4000/auth`).
+  * `client_secret` the client secret copied from the GitHub settings page.

lib/ash_authentication/dsl.ex

@@ -737,7 +737,20 @@ defmodule AshAuthentication.Dsl do
     |> Map.merge(%{
       name: :github,
       args: [{:optional, :name, :github}],
-      describe: "GitHub authentication",
+      describe: """
+      Provides a pre-configured authentication strategy for [GitHub](https://github.com/).
+
+      This strategy is built using `:oauth2` strategy, and thus provides all the same
+      configuration options should you need them.
+
+      For more information see the {{link:ash_authentication:guide:GitHub Quick Start Guide}} in our documentation.
+
+      #### Strategy defaults:
+
+      #{strategy_override_docs(Assent.Strategy.Github)}
+
+      #### Schema:
+      """,
       auto_set_fields: strategy_fields(Assent.Strategy.Github, icon: :github)
     })
   end