finish configuration section

docs/library/Configuration.md

@@ -1,4 +1,24 @@
-# Configuration class
+# Configuration
+
+When using Nominatim through the library, it can be configured in exactly
+the same way as when running as a service. This means that you should have
+created a [project directory](../admin/Import.md#creating-the-project-directory)
+which contains all files belonging to the Noinatim instance. It can also contain
+an `.env` file with configuration options. Setting configuration paramters
+via environment variables works as well.
+
+Configuration options are resolved in the following order:
+
+* from the OS environment (or the dictionary given in `environ`,
+  (see NominatimAPI.md#nominatim.api.core.NominatimAPI.__init__)
+* from the .env file in the project directory of the installation
+* from the default installation in the configuration directory
+
+For more information on configuration via dotenv and a list of possible
+configuration parameters, see the [Configuration page](../customize/Settings.md).
+
+
+## `Configuration` class
 
 ::: nominatim.config.Configuration
     options: