add README's to subfolders within `episodes/`

[avallecam]

projects created from the GitHub template do not have the subfolder structure visible:

• data/
• fig/
• files/

From the {rcompendium} package I learned about the possibility to add READMEs to all subfolders for to objectives:

• provide information to the template user
• make them visible from the beginning on GitHub

Here is an example of what it looks like:

• https://github.com/avallecam/projectname/tree/main/data/raw-data#readme