Notify contributors about transition to Carpentries Workbench

[bielsnohr]

As agreed in a chat with @anenadic :

Tasks

Beta Give feedback

1. We will attempt to get current set of PRs merged in or resolved
2. Any other contributions will be put aside until we go through the template conversion
3. There will be a freeze while the conversion is taking place (around CarpentryConnect)
4. It would be good to send a message to various TopicBox mailing lists to notify people about this
5. Also perhaps a message somewhere on this repo (see PLEASE READ: contribution freeze prior to workbench conversion #391)
6. See if there is a way to notify forks of upcoming changes