Add .env example file.

.env_example

+### Flask ###
+# SECRET_KEY=
+
+### Flask-Mail ###
+MAIL_SERVER=smtp.example.com
+MAIL_PORT=587
+MAIL_USE_TLS=true
+MAIL_USERNAME=username@example.com
+MAIL_PASSWORD=password
+MAIL_DEFAULT_SENDER=username@example.com
+
+### Opaque ###
+OPAQUE_ADMIN=admin.opaque@example.com
+OPAQUE_STORAGE_DIRECTORY=/absolut/path/to/opaque/storage/directory
+
+### Flask-SQLAlchemy ###
+# SQLALCHEMY_DATABASE_URI=