docs(README): adds descriptions for all inputs

[iainsproat]

Description & motivation

Not all inputs to the GitHub action were documented.

Changes:

To-do before merge:

Screenshots:

Validation of changes:

Checklist:

• [ ] My pull request follows the guidelines in the Contributing guide?
• [ ] My pull request does not duplicate any other open Pull Requests for the same update/change?
• [ ] My commits are related to the pull request and do not amend unrelated code or documentation.
• [ ] My code follows a similar style to existing code.
• [ ] I have added appropriate tests.
• [ ] I have updated or added relevant documentation.

References