Update README.md

added mentioning of the `es-7.9.0` branch

Author: lior-k

README.md

@@ -11,6 +11,7 @@ give it a try.
 
 ## Elasticsearch version
 * master branch is designed for Elasticsearch 5.6.9.
+* for Elasticsearch 7.9.0 use branch `es-7.9.0`
 * for Elasticsearch 7.5.0 use branch `es-7.5.0`
 * for Elasticsearch 7.2.1 use branch `es-7.2.1`
 * for Elasticsearch 7.1.0 use branch `es-7.1`