Document environment variables for configuration

docs/configuration.md

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+# Configuration
+
+The variant curation portal can be configured using the following environment variables:
+
+## Web server settings
+
+- `ALLOWED_HOSTS`
+
+  Controls Django's [ALLOWED_HOSTS](https://docs.djangoproject.com/en/2.2/ref/settings/#allowed-hosts) setting.
+  Multiple hosts should be separated with commas. Defaults to `localhost`.
+
+- `SECRET_KEY`
+
+  Controls Django's [SECRET_KEY](https://docs.djangoproject.com/en/2.2/ref/settings/#std:setting-SECRET_KEY) setting.
+  Defaults to a random key generated when the app server starts.
+
+## Database settings
+
+- `DB_ENGINE`
+
+  Controls Django's database [ENGINE](https://docs.djangoproject.com/en/2.2/ref/settings/#engine) setting.
+  Defaults to `django.db.backends.sqlite3`.
+
+- `DB_HOST`
+
+  Controls Django's database [HOST](https://docs.djangoproject.com/en/2.2/ref/settings/#host) setting.
+  Defaults to `localhost`.
+
+- `DB_PORT`
+
+  Controls Django's database [PORT](https://docs.djangoproject.com/en/2.2/ref/settings/#port) setting.
+  Defaults to `5432`.
+
+- `DB_DATABASE`
+
+  Controls Django's database [NAME](https://docs.djangoproject.com/en/2.2/ref/settings/#name) setting.
+  Defaults to a `db.sqlite3` file located in the variant-curation-portal directory.
+
+- `DB_USER`
+
+  Controls Django's database [USER](https://docs.djangoproject.com/en/2.2/ref/settings/#user) setting.
+  Defaults to an empty string.
+
+- `DB_PASSWORD`
+
+  Controls Django's database [PASSWORD](https://docs.djangoproject.com/en/2.2/ref/settings/#password) setting.
+  Defaults to an empty string.
+
+## Authentication settings
+
+- `CURATION_PORTAL_AUTH_HEADER`
+
+  The curation portal uses Django's [RemoteUserMiddleware](https://docs.djangoproject.com/en/2.2/howto/auth-remote-user/)
+  for authentication. This setting controls the header from which the authenticated user's username is read.
+  Defaults to `REMOTE_USER`.