Add top hits aggregation doc.

docs/Aggregation/TopHits.md

+# Top Hits Aggregation
+
+> More info about top hits aggregation is in the [official elasticsearch docs][1]
+
+A top hits metric aggregator keeps track of the most relevant document
+being aggregated. This aggregator is intended to be used as a sub aggregator,
+so that the top matching documents can be aggregated per bucket.
+
+## Simple example
+
+```JSON
+{
+    "aggregation": {
+        "top-tags": {
+            "terms": {
+                "field": "tags",
+                "size": 3
+            },
+            "aggs": {
+                "top_tag_hits": {
+                    "top_hits": {
+                        "sort": [
+                            {
+                                "last_activity_date": {
+                                    "order": "desc"
+                                }
+                            }
+                        ],
+                        "_source": {
+                            "include": [
+                                "title"
+                            ]
+                        },
+                        "size" : 1
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$sort = new Sort('last_activity_date', Sort::ORDER_DESC);
+$topHitsAggregation = new TopHitsAggregation('top_tag_hits', 1, null, $sort);
+$topHitsAggregation->addParameter('_source', ['include' => ['title']]);
+
+$termsAggregation = new TermsAggregation('top-tags', 'tags');
+$termsAggregation->addParameter('size', 3);
+$termsAggregation->addAggregation($topHitsAggregation);
+
+$search = new Search();
+$search->addAggregation($termsAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-top-hits-aggregation.html