Add Percentiles aggregation doc.

docs/Aggregation/Percentiles.md

+# Percentiles Aggregation
+
+> More info about percentiles aggregation is in the [official elasticsearch docs][1]
+
+A multi-value metrics aggregation that calculates one or more percentiles over
+numeric values extracted from the aggregated documents.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "agg_load_time_outlier" : {
+            "percentiles" : {
+                "field" : "load_time"
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$percentilesAggregation = new PercentilesAggregation('load_time_outlier', 'load_time');
+
+$search = new Search();
+$search->addAggregation($percentilesAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-cardinality-aggregation.html