Implements a suggestion by @vdbergh.

We also add a comment in update_json.py to help book authors.

To summarize the CI actions:

• we first run python update_json.py without deleting books.json to quickly flag missing books or wrong sri's
• we then run python update_json.py after deleting books.json , so that all the meta data is computed afresh (this prevents accidental changes to the non-sri fields passing through unnoticed)
• we then run python update_readme.py to ensure integrity of the readme file