mention the Python 3.6 requirement

Author: Vladimir Kotal

opengrok-tools/README.md

@@ -6,6 +6,8 @@ Set of scripts to facilitate project synchronization and mirroring
 The scripts require Python 3 and they rely on a binary/symlink `python3` to be
 present that points to the latest Python 3.x version present on the system.
 
+Currently it is assumed that Python 3.6 and greater is used.
+
 See https://github.com/oracle/opengrok/wiki/Repository-synchronization
 for more details.