Add multi match query doc.

docs/Query/MultiMatch.md

+# Multi Match Query
+
+> More info about multi match query is in the [official elasticsearch docs][1]
+
+The multi match query builds on the [match][2] query to allow multi-field queries:
+
+## Simple example
+
+```JSON
+{
+  "multi_match" : {
+    "query":    "this is a test", 
+    "fields": [ "subject", "message" ] 
+  }
+}
+```
+
+In DSL:
+
+```php
+$multiMatchQuery = new MultiMatchQuery(
+    ['subject', 'message'],
+    'this is a test'
+);
+
+$search = new Search();
+$search->addQuery($multiMatchQuery);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-multi-match-query.html
+[2]: Match.md