docs: add instructions for creating OAuth app with Infomaniak

This commit adds detailed instructions on how to create an OAuth application with Infomaniak to obtain the required Client ID and Client Secret.

Fixes #1

README.md

@@ -33,6 +33,29 @@ You can try out a live demo at [https://demo-oidc.cl1.parano.ch/](https://demo-o
 - Docker (for containerized deployment)
 - An Infomaniak developer account with OIDC client credentials
 
+## Creating an OAuth App with Infomaniak
+
+To use this OIDC validator, you need to create an OAuth application with Infomaniak to obtain a Client ID and Client Secret:
+
+1. Go to [Infomaniak Developer Portal](https://developer.infomaniak.com/)
+2. Sign in with your Infomaniak account or create one if you don't have one
+3. Navigate to "Applications" and click "Create an application"
+4. Fill in the application details:
+   - Name: Choose a name for your application (e.g., "OIDC Validator Demo")
+   - Description: Optional description
+   - Website: Your website URL (can be localhost for development)
+5. In the "Redirect URIs" section, add the callback URL:
+   - For local development: `http://localhost:8000/callback`
+   - For production: `https://your-domain.com/callback`
+6. Select the required scopes:
+   - At minimum, select `openid` and `profile` scopes
+   - You may also want to select `email` scope if you need email information
+7. Save the application
+8. Once created, you'll see your "Client ID" and "Client Secret"
+9. Keep these credentials secure as you'll need them to run the application
+
+Note: For development purposes, you can use `http://localhost:8000` as the redirect URI. For production deployments, make sure to use HTTPS URLs.
+
 ## Installation
 
 ### Local Development