Add On Development: Which files are required in addon directory of a repository?

I am trying to create an addon repository in

It contains the Add on folder “modbus2mqtt”

According to Create an add-on repository | Home Assistant Developer Docs
“To be identified as a repository, the repository must contain a configuration file.”
So, the folder contains a “config.yaml” file.

However, hassio supervisor add on store doesn’t show the add on when adding the repository url GitHub - modbus2mqtt/hassio-modbus2mqtt

The log doesn’t contain useful information.

Are there other files than config.yaml which are required?

Thank you, Volkmar