document python3

Author: Vladimir Kotal

tools/sync/README.md

@@ -68,3 +68,7 @@ projects:
 
 See https://github.com/OpenGrok/OpenGrok/wiki/Per-project-management
 for more details on per-project management.
+
+
+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.