Add prefix query doc.

docs/Query/Prefix.md

+# Prefix Query
+
+> More info about prefix query is in the [official elasticsearch docs][1]
+
+Matches documents that have fields containing terms with a specified prefix.
+
+## Simple example
+
+```JSON
+{
+    "prefix" : { "user" : "ki" }
+}
+```
+
+In DSL:
+
+```php
+$prefixQuery = new PrefixQuery('user', 'ki');
+
+$search = new Search();
+$search->addQuery($prefixQuery);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-prefix-query.html