Add README

README.md

+HAB Solr
+==
+
+This library implements facetted Solr search. It started as a spin-off of [https://github.com/vufind-org](VuFind's
+search component) but simplified the communication with the Solr backend. It implements the
+[https://en.wikipedia.org/wiki/Command_pattern](Command pattern) and provides customizable access to facets. The concept
+of generic *parameter providers* and *response consumers* makes it 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.