docs: adds guide for oauth2 strategy with google (#484)

documentation/tutorials/google-quickstart.md

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+# Google Quick Start Guide
+
+This is a _very quick_ tutorial on how to configure Google authentication in your application using the default oauth2 strategy in ash.
+
+First you'll need a registered application in [Google Cloud](https://console.cloud.google.com/welcome), in order to get your OAuth 2.0 Client credentials.
+
+1. On the Cloud's console **Quick access** section select **APIs & Services**, then **Credentials**
+2. Click on **+ CREATE CREDENTIALS** and from the dropdown select **OAuth client ID**
+3. From the google developers console, we will need: `client_id` & `client_secret`
+
+Next we configure our resource to use google credentials:
+
+``` elixir
+defmodule MyApp.Accounts.User do
+  use Ash.Resource, extensions: [AshAuthentication]
+
+  attributes do
+    ...
+  end
+
+  authentication do
+    strategies do
+      oauth2 :google do
+        client_id "123abc..."
+        redirect_uri {:ok, "123abc..."}
+        client_secret fn -> {:ok, "123abc..."} end
+        site MyApp.Secrets
+      end
+    end
+  end
+end
+```
+
+Please check the guide on how to properly configure your Secrets
+Then we need to define an action that will handle the oauth2 flow, for the google case it is `:register_with_google` it will handle both cases for our resource, user registration & login.
+
+``` elixir
+defmodule MyApp.Accounts.User do
+  use Ash.Resource, extensions: [AshAuthentication]
+  # ...
+  actions do
+    create :register_with_google do
+      argument :user_info, :map, allow_nil?: false
+      argument :oauth_tokens, :map, allow_nil?: false
+      upsert? true
+      upsert_identity :email
+
+      change AshAuthentication.GenerateTokenChange
+
+      # Required if you have the `identity_resource` configuration enabled.
+      # change AshAuthentication.Strategy.OAuth2.IdentityChange
+
+      change fn changeset, _ctx ->
+        user_info = Ash.Changeset.get_argument(changeset, :user_info)
+        changeset
+          |> Ash.Changeset.change_attribute(:google_info, user_info)
+          # you could upsert custom user attributes from the given google's user_info
+          # |> Ash.Changeset.change_attribute(:email, user_info["email"])
+          # |> Ash.Changeset.change_attribute(:name, user_info["name"])
+          # |> Ash.Changeset.change_attribute(:portrait, user_info["picture"])
+      end
+    end
+  end
+  # ...
+end
+```