added documentation

docs/Aggregation/Pipeline/SumBucket.md

+# Sum Bucket Aggregation
+
+> More info about sum bucket aggregation is in the [official elasticsearch docs][1]
+
+A sibling pipeline aggregation which calculates the sum across all bucket of a 
+specified metric in a sibling aggregation. The specified metric must be numeric and 
+the sibling aggregation must be a multi-bucket aggregation.
+
+## Simple example
+
+```JSON
+{
+    "aggs" : {
+        "sales_per_month" : {
+            "date_histogram" : {
+                "field" : "date",
+                "interval" : "month"
+            },
+            "aggs": {
+                "sales": {
+                    "sum": {
+                        "field": "price"
+                    }
+                }
+            }
+        },
+        "sum_monthly_sales": {
+            "sum_bucket": {
+                "buckets_path": "sales_per_month>sales" 
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$dateAggregation = new DateHistogramAggregation('sales_per_month', 'date', 'month');
+$dateAggregation->addAggregation(new SumAggregation('sales', 'price'));
+$sumBucketAggregation = new SumBucketAggregation(
+    'sum_monthly_sales', 
+    'sales_per_month>sales'
+)
+
+$search = new Search();
+$search->addAggregation($dateAggregation);
+$search->addAggregation($sumBucketAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-pipeline-sum-bucket-aggregation.html