Add nested query doc.

docs/Query/Nested.md

+# Nested Query
+
+> More info about nested query is in the [official elasticsearch docs][1]
+
+Nested query allows to query nested objects / documents (see [nested mapping][2]). The query is executed against the nested
+objects / documents as if they were indexed as separate documents (they are, internally) and resulting in the root
+parent document (or parent nested mapping).
+
+## Simple example
+
+```JSON
+{
+    "nested" : {
+        "path" : "obj1",
+        "score_mode" : "avg",
+        "query" : {
+            "bool" : {
+                "must" : [
+                    {
+                        "match" : {"obj1.name" : "blue"}
+                    },
+                    {
+                        "range" : {"obj1.count" : {"gt" : 5}}
+                    }
+                ]
+            }
+        }
+    }
+}
+```
+
+In DSL:
+
+```php
+$matchQuery = new MatchQuery('obj1.name', 'blue');
+$rangeQuery = new RangeQuery('obj1.count', ['gt' => 5]);
+$boolQuery = new BoolQuery();
+$boolQuery->add($matchQuery);
+$boolQuery->add($rangeQuery);
+
+$nestedQuery = new NestedQuery(
+    'obj1',
+    $boolQuery
+);
+$nestedQuery->addParameter('score_mode', 'avg');
+
+$search = new Search();
+$search->addQuery($nestedQuery);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-nested-query.html
+[2]: https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-nested-type.html