Add missing filter doc.

docs/Filter/Missing.md

+# Missing Filter
+
+> More info about missing filter is in the [official elasticsearch docs][1]
+
+Returns documents that have only null values or no value in the original field.
+
+## Simple example
+
+```JSON
+{
+    "filter" : {
+        "missing" : { "field" : "user" }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$missingFilter = new MissingFilter('user');
+
+$search = new Search();
+$search->addFilter($missingFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: hhttps://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-missing-filter.html