Add references to datasets in README

[luiskuhn]

In the “core” repo, add reference to training dataset in zenodo, in pred pkg add ref. to pypi package page, and in rta add ref to “full” dataset in zenodo

[luiskuhn]

Solved by:

• https://github.com/qbic-pipelines/rts-prediction-package/pull/148
• https://github.com/qbic-pipelines/root-tissue-analysis/pull/21
• https://github.com/qbic-pipelines/root-tissue-segmentation-core/pull/5