Add cardinality aggregation doc.

docs/Aggregation/Cardinality.md

+# Cardinality Aggregation
+
+> More info about cardinality aggregation is in the [official elasticsearch docs][1]
+
+A single-value metrics aggregation that calculates an approximate count of distinct values.
+
+## Simple example
+
+```JSON
+{
+     "aggregations" : {
+         "agg_author_count" : {
+             "cardinality" : {
+                 "field" : "author"
+             }
+         }
+     }
+ }
+```
+
+And now the query via DSL:
+
+```php
+$cardinalityAggregation = new CardinalityAggregation('author_count', 'author');
+
+$search = new Search();
+$search->addAggregation($cardinalityAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-cardinality-aggregation.html