Add regexp filter doc.

docs/Filter/Regexp.md

+# Regexp Filter
+
+> More info about regexp filter is in the [official elasticsearch docs][1]
+
+The regexp filter allows you to use regular expression term queries.
+The regexp filter is similar to the [regexp query][2],
+except that it is cacheable and can speedup performance in case you are
+reusing this filter in your queries.
+
+## Simple example
+
+```JSON
+{
+    "regexp":{
+        "name.first": "s.*y"
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$regexpFilter = new RegexpFilter('name.first', 's.*y');
+
+$search = new Search();
+$search->addFilter($regexpFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-indices-filter.html
+[2]: ../Query/Regexp.md
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