add code structure to README

README.md

@@ -420,6 +420,22 @@ Generate the migration script and make sure to review it before commit it. Somet
 
 > flask db migrate
 
+### Code structure
+
+The repo consists of 3 entry points:
+
+- wsgi.py and server.py: the webapp. 
+- email_handler.py: the email handler. 
+- cron.py: the cronjob. 
+
+Here are a small sum-ups of the directory structure and their roles:
+
+- app/: main Flask app. It is divided into different packages that represents different features like oauth,  api, dashboard, etc.
+- local_data/: contain files that are there to facilitate local development. They are replaced during the deployment.
+- migrations/: generated by flask-migrate. Edit these files only when you spot (very rare) errors on database migration files.
+- static/: files available at `/static` url. 
+- templates/: contain both html and email templates.
+- tests/: tests. We don't really distinguish unit, functional or integration test. A test is simply here to make sure a feature works correctly.
 
 ### OAuth flow