docs: added CONFIGURATION.md

Author: mvegter

docs/CONFIGURATION.md

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+# Configuration
+
+## Database
+| Variable name | Description | Default value |
+|---------------|-------------|---------------|
+| DATABASE_HOST | The host of the relational database. | localhost |
+| DATABASE_PORT | The port of the relational database. | 3306 |
+| DATABASE_USERNAME | The username which is used to authenticate against the database. | cern |
+| DATABASE_PASSWORD | The password which is used to authenticate against the database. | cern |
+| DATABASE_NAME | The name of the database | bookkeeping |