Add date range aggregation doc.

docs/Aggregation/DateRange.md

+# Date Range Aggregation
+
+> More info about date range aggregation is in the [official elasticsearch docs][1]
+
+A range aggregation that is dedicated for date values.
+
+## Simple example
+
+```JSON
+{
+    "aggregations": {
+        "agg_range": {
+            "date_range": {
+                "field": "date",
+                "format": "MM-yyy",
+                "ranges": [
+                    { "to": "now-10M/M" }, 
+                    { "from": "now-10M/M" } 
+                ]
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$dateRangeAggregation = new DateRangeAggregation('range');
+$dateRangeAggregation->setField('date');
+$dateRangeAggregation->setFormat('MM-yyy');
+$dateRangeAggregation->addRange(null, 'now-10M/M');
+$dateRangeAggregation->addRange('now-10M/M', null);
+```
+
+Or : 
+
+```php
+$dateRangeAggregation = new DateRangeAggregation(
+    'range',
+    'date',
+    'MM-yyy',
+    [
+        ['to' => 'now-10M/M'],
+        ['from' => 'now-10M/M'],
+    ]
+);
+
+$search = new Search();
+$search->addAggregation($dateRangeAggregation);
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-daterange-aggregation.html