Add indices filter doc.

docs/Filter/Indices.md

+# Indices Filter
+
+> More info about indices filter is in the [official elasticsearch docs][1]
+
+The indices filter can be used when executed across multiple indices,
+allowing to have a filter that executes only when executed on an index
+that matches a specific list of indices, and another filter that
+executes when it is executed on an index that does not match the listed
+indices.
+
+## Simple example
+
+```JSON
+{
+    "indices" : {
+        "indices" : ["index1", "index2"],
+        "filter" : {
+            "term" : { "tag" : "wow" }
+        },
+        "no_match_filter" : {
+            "term" : { "tag" : "kow" }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$termFilter1 = new TermFilter('tag', 'wow');
+$termFilter2 = new TermFilter('tag', 'kow');
+
+$indicesFilter = new IndicesFilter(
+    ['index1', 'index2'],
+    $termFilter1,
+    $termFilter2
+);
+
+$search = new Search();
+$search->addFilter($indicesFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-indices-filter.html