added documentation

docs/Aggregation/Pipeline/CumulativeSum.md

+# Cumulative Sum Aggregation
+
+> More info about cumulative sum aggregation is in the [official elasticsearch docs][1]
+
+A parent pipeline aggregation which calculates the cumulative sum of a specified metric 
+in a parent histogram (or date_histogram) aggregation. The specified metric must be numeric 
+and the enclosing histogram must have min_doc_count set to 0 (default for histogram 
+aggregations).
+
+## Simple example
+
+```JSON
+{
+    "aggs" : {
+        "sales_per_month" : {
+            "date_histogram" : {
+                "field" : "date",
+                "interval" : "month"
+            },
+            "aggs": {
+                "sales": {
+                    "sum": {
+                        "field": "price"
+                    }
+                },
+                "cumulative_sales": {
+                    "cumulative_sum": {
+                        "buckets_path": "sales" 
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$search = new Search();
+
+$dateAggregation = new DateHistogramAggregation('sales_per_month', 'date', 'month');
+$dateAggregation->addAggregation(
+    new SumAggregation('sales', 'price')
+);
+$dateAggregation->addAggregation(
+    new CumulativeSumAggregation('cumulative_sales', 'sales')
+);
+
+$search->addAggregation($dateAggregation);
+
+$aggArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-pipeline-cumulative-sum-aggregation.html
\ No newline at end of file