Add match all filter doc.

docs/Filter/MatchAll.md

+# Match All Filter
+
+> More info about match all filter is in the [official elasticsearch docs][1]
+
+A filter that matches on all documents.
+
+## Simple example
+
+```JSON
+{
+    "filter" : {
+        "match_all" : { }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$matchAllFilter = new MatchAllFilter();
+
+$search = new Search();
+$search->addFilter($matchAllFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-indices-filter.html