add r_make example

[boshek]

Summary

Offered to provide an example of using r_make for within target parallelization.

Checklist

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

[boshek]

Linked to the r_make section in the manual as well as provided the reason. HTH.