Add exists filter doc.

docs/Filter/Exists.md

+# Exists Filter
+
+> More info about exists filter is in the [official elasticsearch docs][1]
+
+Returns documents that have at least one non-null value in the original field.
+
+## Simple example
+
+```JSON
+{
+    "constant_score" : {
+        "filter" : {
+            "exists" : { "field" : "user" }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$existsFilter = new ExistsFilter('user');
+$constantScoreQuery = new ConstantScoreQuery($existsFilter);
+
+$search = new Search();
+$search->addQuery($constantScoreQuery);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-exists-filter.html