Add readme for custom example

examples/custom/README.md

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+# authkit2 - Custom Integration Example
+
+This folder contains an example that demonstrates using authkit2 for
+authenticating against a OIDC provider outside of the Laravel framework.
+
+# Setup
+
+Install dependencies (only authkit2) with:
+
+```
+$ composer install
+```
+
+Configure your application's client credentials and redirect URLs in
+`_config.php`.
+
+Note that the OpenID endpoint must include the full path to the realm, e.g. for
+Keycloak, `http://127.0.0.1:8080/auth/realms/Test`
+
+# Usage
+
+You can serve this project with PHP's integrated development server. From this
+folder, run:
+
+```
+$ php -S 127.0.0.1:4444
+```
+
+Then access it in your browser at [http://127.0.0.1:4444](http://127.0.0.1:4444/).
+
+Don't forget to register your callback URLs with your OpenID provider.
+
+# Layout
+
+* `_config.php`: Constants to define your configuration.
+* `_common.php`: Common code: initialize session, pass configuration to
+  authkit, helpers for rendering page header/footer
+* `index.php`: Basic page to show logged in user's information or provide a
+  prompt to login.
+* `logout.php`: Page to log the user out, redirect them to the provider to
+  logout, and back to index.
+* `login.php`: Initiate the login process and redirect the user to the provider.
+* `callback.php`: Where the provider redirects the user back to, receives the
+  nonce and authorization code, exchanges those for a token, and redirects the
+  user back to the index.
+