Add development setup guide

DEVELOPMENT.md

+# Development Guide
+
+## Setup
+
+* Install python3 and virtualenv
+* Create a virtualenv and install the requirements
+
+```
+virtualenv -p python3 venv
+source venv/bin/activate
+pip install -r requirements.txt
+```
+
+* Create a database (with sqlite, postgres, …)
+* Create a config file
+
+```
+cp config.py.example config.py
+```
+
+* Edit the config file to your needs
+  - Don't forget to enter your database connection
+  - And your authentication backend
+  - And deactivate everything you do not use
+
+* Fill your database
+
+```
+./server.py db upgrade
+```
+
+## Running the program
+
+```
+source venv/bin/activate
+./server.py runserver
+```
+
+The website will run on `localhost:5000`.
+
+

config.py.example

 # (local) database
 SQLALCHEMY_DATABASE_URI = "postgresql://user:password@host/database" # change this
+#SQLALCHEMY_DATABASE_URI = "mysql://user:password@host/database"
+#SQLALCHEMY_DATABASE_URI = "sqlite:///path/to/database.db"
 SQLALCHEMY_TRACK_MODIFICATIONS = False # do not change
 
 SECRET_KEY = "something random" # change this