docs: add a link to the sd_model vignette and review text

- README
- airGR.Rd
Refs #114
3 jobs for 90-fix-incorrect-parameter-transformation-for-x5-in-gr5j
Status Name Job ID Coverage
  Check
canceled check_as_cran_patched #167226

canceled check_not_cran_patched #167225

 
  Regression
canceled regression_patched #167227