added documentation

docs/Aggregation/Pipeline/BucketSelector.md

+# Bucket Selector Aggregation
+
+> More info bucket selector aggregation is in the [official elasticsearch docs][1]
+
+A parent pipeline aggregation which executes a script which determines whether the 
+current bucket will be retained in the parent multi-bucket aggregation. The specified 
+metric must be numeric and the script must return a boolean value. If the script 
+language is expression then a numeric return value is permitted. In this case 0.0 will 
+be evaluated as false and all other values will evaluate to true.
+
+## Simple example
+
+```JSON
+{
+    "aggs" : {
+        "sales_per_month" : {
+            "date_histogram" : {
+                "field" : "date",
+                "interval" : "month"
+            },
+            "aggs": {
+                "total_sales": {
+                    "sum": {
+                        "field": "price"
+                    }
+                },
+                "sales_bucket_filter": {
+                    "bucket_selector": {
+                        "buckets_path": {
+                          "totalSales": "total_sales"
+                        },
+                        "script": "totalSales <= 50"
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$search = new Search();
+
+$dateAggregation = new DateHistogramAggregation('sales_per_month', 'date', 'month');
+$dateAggregation->addAggregation(
+    new SumAggregation('total_sales', 'price');
+);
+$scriptAggregation = new BucketSelectorAggregation(
+    'sales_bucket_filter',
+    ['totalSales' => 'total_sales']
+);
+$scriptAggregation->setScript('totalSales <= 50');
+$dateAggregation->addAggregation($scriptAggregation);
+
+$search->addAggregation($dateAggregation);
+
+$aggArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-pipeline-bucket-selector-aggregation.html
\ No newline at end of file