More info about GitLab integrations with OAuth

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/integration/oauth.md

+# OAuth
+
 You can use other services to log into GitLab via oAuth.
 
-Please configure this in gitlab.yml
+For this you need: 
+
+* create app in selected services
+* configure gitlab.yml 
+
+## Twitter:
 
 Below are screenshots how to setup your app on Twitter for this:
 
 ![Application details](twitter_app_details.png)
 ![API Keys](twitter_app_api_keys.png)
 
-And GitHub:
+## GitHub:
 
 ![GitHub app](github_app.png)
+
+## Google:
+
+![Google app](google_app.png)
+
+
+## GitLab config file
+
+Second step is to modify gitlab.yml with app credentials:
+
+```
+production:
+  ...
+
+  omniauth:
+    enabled: true
+  
+    providers:
+      - { 
+        name: 'twitter',
+        app_id: 'XXXXXXXX',
+        app_secret: 'XXXXXXXXXXXXXXXXXXXXXXXX'
+        }
+      - { 
+        name: 'google_oauth2',
+        app_id: 'XXXXXXXXXXX.apps.googleusercontent.com',
+        app_secret: 'XXXXXXXX'
+        }
+      - { 
+        name: 'github',
+        app_id: 'XXXXXXXXXX',
+        app_secret: 'XXXXXXXXXXXXXXXXXXXXXXXX'
+        }
+
+```
+