Add stats aggregation doc.

docs/Aggregation/Stats.md

+# Stats Aggregation
+
+> More info about 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" : { "stats" : { "field" : "grade" } }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$statsAggregation = new StatsAggregation('grades_stats', 'grade');
+
+$search = new Search();
+$search->addAggregation($statsAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-stats-aggregation.html