This is the list of Solidus extensions that we currently manage:
https://docs.google.com/spreadsheets/d/1DsER7GfOE98jTYFvoQULYzsbiTEEhmrZqsEfUaLFoaQ/edit#gid=0
The file README.example.md includes a template for extension README with suggestions and conventions.
This page has some hints on releasing a new version of Solidus. Similar conditions should apply for the extensions.