added documentation for the aggregation

docs/Aggregation/DateHistogram.md

+# Date Histogram Aggregation
+
+> More info about histogram aggregation is in the [official elasticsearch docs][1]
+
+A multi-bucket aggregation similar to the histogram except it can only be applied on date values.
+Example of expressions for interval: `year`, `quarter`, `month`, `week`, `day`, `hour`, `minute`, `second`
+
+## Simple example
+
+```JSON
+{
+    "aggregations": {
+        "articles_over_time" : {
+            "date_histogram" : {
+                "field" : "date",
+                "interval" : "month"
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$dateHistogramAggregation = new DateHistogramAggregation('articles_over_time', 'date', 'month');
+
+$search = new Search();
+$search->addAggregation($dateHistogramAggregation);
+
+$queryArray = $search->toArray();
+```
+
+## Adding parameters example
+
+Additional parameters can be added to the aggregation. In the following example we will demonstrate how
+to provide a custom format to the results of the query:
+
+```JSON
+{
+    "aggregations": {
+        "articles_over_time" : {
+            "date_histogram" : {
+                "field" : "date",
+                "interval" : "1M",
+                "format" : "yyyy-MM-dd"
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+
+$dateHistogramAggregation = new DateHistogramAggregation('articles_over_time', 'date', 'month');
+$dateHistogramAggregation->addParameter('format', 'yyyy-MM-dd');
+
+$search = new Search();
+$search->addAggregation($dateHistogramAggregation);
+
+$queryArray = $search->toArray();
+
+```
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-datehistogram-aggregation.html
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