Add script filter doc.

docs/Filter/Script.md

+# Script Filter
+
+> More info about script filter is in the [official elasticsearch docs][1]
+
+A filter allowing to define scripts as filters
+
+## Simple example
+
+```JSON
+{
+    "filter" : {
+        "script" : {
+            "script" : "doc['num1'].value > 1"
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$scriptFilter = new ScriptFilter('doc[\'num1\'].value > 1');
+
+$search = new Search();
+$search->addFilter($scriptFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-script-filter.html