Add Percentile Ranks aggregation doc.

docs/Aggregation/PercentileRanks.md

+# Percentile Ranks Aggregation
+
+> More info about percentile ranks aggregation is in the [official elasticsearch docs][1]
+
+A multi-value metrics aggregation that calculates one or more percentile
+ranks over numeric values extracted from the aggregated documents.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "agg_load_time_outlier" : {
+            "percentile_ranks" : {
+                "field" : "load_time",
+                "values" : [15, 30]
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$percentileRanksAggregation = new PercentileRanksAggregation('load_time_outlier', 'load_time', [15, 30]);
+
+$search = new Search();
+$search->addAggregation($percentileRanksAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-percentile-rank-aggregation.html