Add sum aggregation doc.

docs/Aggregation/Sum.md

+# Sum Aggregation
+
+> More info about sum aggregation is in the [official elasticsearch docs][1]
+
+A single-value metrics aggregation that sums up numeric values that are extracted from the aggregated documents.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "agg_intraday_return" : { "sum" : { "field" : "change" } }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$sumAggregation = new SumAggregation('intraday_return', 'change');
+
+$search = new Search();
+$search->addAggregation($sumAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-sum-aggregation.html