Add indices query doc.

docs/Query/Indices.md

+# Indices Query
+
+> More info about indices query is in the [official elasticsearch docs][1]
+
+The indices query can be used when executed across multiple indices, allowing to have a query that executes
+only when executed on an index that matches a specific list of indices, and another query that executes
+when it is executed on an index that does not match the listed indices.
+
+## Simple example
+
+```JSON
+{
+    "indices" : {
+        "indices" : ["index1", "index2"],
+        "query" : {
+            "term" : { "tag" : "wow" }
+        },
+        "no_match_query" : {
+            "term" : { "tag" : "kow" }
+        }
+    }
+}
+```
+
+In DSL:
+
+```php
+$matchTermQuery = new TermQuery('tag', 'wow');
+$noMatchTermQuery = new TermQuery('tag', 'kow');
+
+$indicesQuery = new IndicesQuery(['index1', 'index2'], $matchTermQuery, $noMatchTermQuery);
+
+$search = new Search();
+$search->addQuery($indicesQuery);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-indices-query.html