DATAKV-310 - Polishing.

Use consistent punctuation in asserts.
Update link to code of conduct.

Original Pull Request: #50

src/main/java/org/springframework/data/map/MapKeyValueAdapter.java

@@ -188,7 +188,7 @@ public class MapKeyValueAdapter extends AbstractKeyValueAdapter {
 	 */
 	protected Map<Object, Object> getKeySpaceMap(String keyspace) {
 
-		Assert.notNull(keyspace, "Collection must not be null for lookup.");
+		Assert.notNull(keyspace, "Collection must not be null for lookup!");
 		return store.computeIfAbsent(keyspace, k -> CollectionFactory.createMap(keySpaceMapType,  1000));
 	}