Add value count aggregation doc.

docs/Aggregation/ValueCount.md

+# Value Count Aggregation
+
+> More info about value count aggregation is in the [official elasticsearch docs][1]
+
+A single-value metrics aggregation that counts the number of values that are extracted from the aggregated documents.
+
+## Simple example
+
+```JSON
+{
+    "aggs" : {
+        "grades_count" : { "value_count" : { "field" : "grade" } }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$valueCountAggregation = new ValueCountAggregation('grades_count', 'grade');
+
+$search = new Search();
+$search->addAggregation($valueCountAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-valuecount-aggregation.html