make clear dynamic branching only in dev version

[mattm-dhsc]

The current CRAN 'r-release' v 7.7.0 does not have dynamic branching and this is the version many users would install. Led to a little bit of head scratching when running the code in the example didn't do what the book showed it did!

Summary

Proposed change to text of the book to distinguish between r-release & r-devel

Related GitHub issues and pull requests

None

Checklist

• [ X] I understand and agree to this repository's code of conduct.
• [ X] I have listed any substantial changes in the development news.
• [X ] This pull request is not a draft.

[wlandau]

Hi @mattm-dhsc, sorry about the confusion. I am planning on a CRAN release fairly soon (hopefully within a week or two) so this ambiguity is only temporary. Could we wait until then? Otherwise, I will probably forget to remove the note about the development version.

[wlandau]

Just released version 7.8.0 to CRAN, which has dynamic branching.