add information for updating TEIGarage to TCW22

[peterstadler]

During the release process a new TEIGarage release needs to be done (for the TEIGarage Docker image includes the TEI Stylesheets).

@anneferger and I envision a GitHub action in the TEIGarage repo that will be triggered by a new Git tag and a dedicated commit message, see TEIC/TEIGarage#86.

Once this is set up we'll need to add the exact procedure to TCW22. @anneferger volunteered to add this :)