Add nested aggregation doc.

docs/Aggregation/Nested.md

+# Nested Aggregation
+
+> More info about nested aggregation is in the [official elasticsearch docs][1]
+
+A special single bucket aggregation that enables aggregating nested documents.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "agg_resellers" : {
+            "nested" : {
+                "path" : "resellers"
+            },
+            "aggregations" : {
+                "agg_min_price" : { "min" : { "field" : "resellers.price" } }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$minAggregation = new MinAggregation('min_price', 'resellers.price');
+$nestedAggregation = new NestedAggregation('resellers', 'resellers');
+$nestedAggregation->addAggregation($minAggregation);
+
+$search = new Search();
+$search->addAggregation($nestedAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-nested-aggregation.html