added documentation

docs/Aggregation/Pipeline/Derrivative.md

+# Derivative Aggregation
+
+> More info about derivative aggregation is in the [official elasticsearch docs][1]
+
+A parent pipeline aggregation which calculates the derivative 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"
+                    }
+                },
+                "sales_deriv": {
+                    "derivative": {
+                        "buckets_path": "sales" 
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$dateAggregation = new DateHistogramAggregation('sales_per_month', 'date', 'month');
+$sumAggregation = new SumAggregation('sales', 'price');
+$derivativeAggregation = new DerivativeAggregation('sales_deriv', 'sales');
+$dateAggregation->addAggregation($sumAggregation);
+$dateAggregation->addAggregation($derivativeAggregation);
+
+$search = new Search();
+$search->addAggregation($dateAggregation);
+
+$queryArray = $search->toArray();
+```
+
+## Second order derivative
+
+Somewhat more complex would be an example of a second order derivatives. This functionality
+is presented in the folowing example:
+
+```json
+{
+    "aggs" : {
+        "sales_per_month" : {
+            "date_histogram" : {
+                "field" : "date",
+                "interval" : "month"
+            },
+            "aggs": {
+                "sales": {
+                    "sum": {
+                        "field": "price"
+                    }
+                },
+                "sales_deriv": {
+                    "derivative": {
+                        "buckets_path": "sales"
+                    }
+                },
+                "sales_2nd_deriv": {
+                    "derivative": {
+                        "buckets_path": "sales_deriv" 
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+And now via DSL:
+
+```php
+$dateAggregation = new DateHistogramAggregation('sales_per_month', 'date', 'month');
+$sumAggregation = new SumAggregation('sales', 'price');
+$firstDerivativeAggregation = new DerivativeAggregation('sales_deriv', 'sales');
+$secondDerivativeAggregation = new DerivativeAggregation('sales_2nd_deriv', 'sales_deriv');
+
+$dateAggregation->addAggregation($sumAggregation);
+$dateAggregation->addAggregation($firstDerivativeAggregation);
+$dateAggregation->addAggregation($secondDerivativeAggregation);
+
+$search = new Search();
+$search->addAggregation($dateAggregation);
+
+$queryArray = $search->toArray();
+
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-pipeline-derivative-aggregation.html