RIBU stands for Read Instructions Before Use.

This tells the user to read the instructions (e.g. README, PDF, manual, etc.) before opening up the main file.

The main file does not have to end with .ribu it just tells the user to read the "tutorial" before opening up not just the main file but possibly/optionally any file.

It is not mandatory but I would really appreciate it if you give credit to me if you use or incorporate a .ribu file

Once the main_RIBU.ribu file is downloaded, you now have the extension.