Add terms filter doc.

docs/Filter/Terms.md

+# Terms Filter
+
+> More info about terms filter is in the [official elasticsearch docs][1]
+
+Filters documents that have fields that match any of the provided terms.
+
+## Simple example
+
+```JSON
+{
+    "filter" : {
+       "terms" : { "user" : ["kimchy", "elasticsearch"]}
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$termsFilter = new TermsFilter('user', ['kimchy', 'elasticsearch']);
+
+$search = new Search();
+$search->addFilter($termsFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-terms-filter.html