Add paper ref to README

We can add the [ImagNeur ref](https://direct.mit.edu/imag/article/doi/10.1162/IMAG.a.123/132199/Rootlets-based-registration-to-the-PAM50-spinal?searchresult=1) to the README, for example as done for the rootlets repo: https://github.com/ivadomed/model-spinal-rootlets/blob/main/README.md