Update README

README.md

@@ -8,3 +8,22 @@ easy to implemented access to Solr features not covered by this library.
 
 HAB Solr is Copyright (c) 2016-2019 by Herzog August Bibliothek Wolfenbüttel and released under the terms of the GNU
 General Public License v3 or higher.
+
+Using the invoker
+--
+
+The [Invoker](src/HAB/Solr/Command/Search.php) reads the query parameters from the [Command](src/HAB/Solr/Command/Search.php), 
+optionally merges them with static or dynamic default parameters, and communicates with the Solr backend using a
+[Guzzle](http://guzzlephp.org) HTTP client.
+
+The body of the Solr response is passed to [Command](src/HAB/Solr/Command/Search.php).
+
+Using the search command
+--
+
+The [Search](src/HAB/Solr/Command/Search.php) command performs a search on a Solr query handler. 
+
+The search query is represented by an ArrayObject and translated into a Solr search query by a
+[QueryBuilder](src/HAB/Solr/Command/QueryBuilder.php). The default implementation performs a 1:1 mapping of the
+ArrayObject's keys and values to Solr search parameters.
+