Update documentation.

Original Pull Request: #654

Author: christophstrobl

src/main/antora/modules/ROOT/pages/keyvalue/repository/map-repositories.adoc

@@ -23,3 +23,36 @@ It is possible to change the `QueryEngine` and use a custom one instead of the d
 The `EnableMapRepositories` annotation allows to configure the by supplying a `QueryEngineFactory` as well as the `QueryCreator` via according attributes.
 Please mind that the `QueryEngine` needs to be able to process queries created by the configured `QueryCreator`.
 
+== Storage Backend Configuration
+
+`KeySpaceStore` provides a simple storage facade that can be used along with the `keySpaceStoreRef` attribute of `EnableMapRepositories` to set the bean reference to the actual storage, overriding `mapType` settings.
+
+====
+[source,java]
+----
+@Configuration
+@EnableMapRepositories(keySpaceStoreRef = "store") <1>
+public class MapDbConfiguration {
+
+	@Bean
+	KeySpaceStore store(DB db) { <1>
+
+		return new KeySpaceStore() {
+
+			@Override
+			public Map<Object, Object> getKeySpace(String keyspace) {
+				return db.hashMap(keyspace, ...
+			}
+
+			@Override
+			public void clear() {
+				//
+			}
+
+		};
+	}
+
+}
+----
+<1> reference the `KeySpaceStore` by bean name.
+====