Thanks @jimhester. I am trying this or a modified version now.
In investigating why the workflow isn't showing up, I came across a solution to another issue in the GitHub Community site where it was stated that a workflow has to be triggered before it will show up in the Actions tab. Changing the on: part to trigger on any push to my master branch triggered the workflow and the workflow showed up in the list.
I'm not sure that the current behaviour of the workflow is the correct one for rendering a README.Rmd, certainly not in my use case. I can see the logic of triggering the workflow only when README.Rmd is included in a commit pushed to a repo. However, if the README.Rmd is unchanged but the code it runs has changed due to other commits, unless one needlessly edits and pushes the README.Rmd the workflow won't trigger and the README.md won't get updated. In my use case I may change the code in the package that is called in the README.Rmd and I want that to be reflected in the README.md even if I don't edit README.Rmd.
Might be worth adding a comment to the .yaml indicating that this workflow will only trigger if README.Rmd is part of a commit/push. And suggest an alternative on: if the user wants to rebuild the README.Rmd on every push.?