Add prefix filter doc.

docs/Filter/Prefix.md

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