Add Histogram aggregation doc.

docs/Aggregation/Histogram.md

+# Histogram Aggregation
+
+> More info about histogram aggregation is in the [official elasticsearch docs][1]
+
+A multi-bucket values source based aggregation that can be applied on numeric values extracted from
+the documents. It dynamically builds fixed size (a.k.a. interval) buckets over the values.
+
+## Simple example
+
+```JSON
+{
+    "aggregations": {
+        "agg_prices": {
+            "histogram": {
+                "field": "price",
+                "interval": 50
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$histogramAggregation = new HistogramAggregation('prices', 'price', 50);
+
+$search = new Search();
+$search->addAggregation($histogramAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-histogram-aggregation.html