Add Google auth documentation

docs/google-authentication.markdown

@@ -0,0 +1,65 @@
+Google Authentication
+=====================
+
+Requirements
+------------
+
+- OAuth Google API credentials (available in the Google Developer Console)
+
+How does this work?
+-------------------
+
+- The Google authentication in Kanboard use the OAuth 2.0 protocol
+- Any user account in Kanboard can be linked to a Google Account
+- When a Kanboard user account is linked to Google, you can login with one click
+
+Procedure to link a Google Account
+----------------------------------
+
+1. The first step is to link an existing user account to a Google Account
+2. Login with the desired user
+3. Go to the **Edit user** page and click on the link **Link my Google Account**
+4. You are redirected to the **Google Consent screen**, authorize Kanboard by clicking on the button **Accept**
+5. Finally, you are redirected to Kanboard and now your user account is linked to your Google account
+6. During the process, Kanboard have updated your full name and your email address based on your Google profile
+7. Log out and you should be able to login directly with Google by clicking on the link **Login with my Google Account**
+
+Installation instructions
+-------------------------
+
+### Setting up OAuth 2.0
+
+Follow the [official Google documentation](https://developers.google.com/accounts/docs/OAuth2Login#appsetup), in summary:
+
+- Go to the [Developer Console](https://console.developers.google.com)
+- On the sidebar, click on **Credentials** and choose **Create a new Client Id**
+- Fill the form, the redirect URL must be: **http://YOUR_SERVER/?controller=user&action=google**
+
+### Setting up Kanboad
+
+Create a custom `config.php` file or copy the `config.default.php` file:
+
+```php
+<?php
+
+// Enable/disable Google authentication
+define('GOOGLE_AUTH', true); // Set this value to true
+
+// Google client id (Get this value from the Google developer console)
+define('GOOGLE_CLIENT_ID', 'YOUR_CLIENT_ID');
+
+// Google client secret key (Get this value from the Google developer console)
+define('GOOGLE_CLIENT_SECRET', 'YOUR_CLIENT_SECRET');
+
+```
+
+Notes
+-----
+
+Kanboard use these information from your Google profile:
+
+- Full name
+- Email address
+- Google unique id
+
+The Google unique id is used to link together the local user account and the Google account.