Add extended stats aggregation doc.

docs/Aggregation/ExtendedStats.md

+# Extended Stats Aggregation
+
+> More info about extended stats aggregation is in the [official elasticsearch docs][1]
+
+A multi-value metrics aggregation that computes stats over numeric values extracted from the aggregated documents.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "agg_grades_stats" : {
+            "extended_stats" : { "field" : "grade" }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$extendedStatsAggregation = new ExtendedStatsAggregation('grades_stats', 'grade');
+
+$search = new Search();
+$search->addAggregation($extendedStatsAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-extendedstats-aggregation.html