Add max aggregation doc.

docs/Aggregation/Max.md

+# Max Aggregation
+
+> More info about max aggregation is in the [official elasticsearch docs][1]
+
+A single-value metrics aggregation that keeps track and returns the
+maximum value among the numeric values extracted from the aggregated documents.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "max_price" : { "max" : { "field" : "price" } }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$maxAggregation = new MaxAggregation('max_price', 'price');
+
+$search = new Search();
+$search->addAggregation($maxAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-max-aggregation.html